Rohault's system of natural philosophy

THE

Tranjlatofs Preface.

lE feveralEditions which thís
Treatife has pafs’d through,
both in French and Latin,
are a fuffident Teftimony
how acceptable and . ufeful
it has been to the World,
and a juft Apology for my
tranílating it into EngUfo. \ fhall not
therefore trouble the Reader with any par-
ticular Account eithcr of the Excellency of
the Subjeft, the Abilitiesof the Author,or the
Method he has proceeded in, but refer theni
all to be judged of by the Book it felf : Only
as to the Notes the Reader is defired to take
Notice, that therein is a full Anfwer to fuch
Objedions made againft the Author as feeni
not to have any juft Foundation, and a grcat
inany Things in Natural Philofophy, which
have been lince found out by the Pains and
Induftry of later Philofophers, are here fe-
le¿ted frorat^/beft Writers i and there are
A 2 alfo

The Ttanslato r’s T RE FAC E,

alio feveral Thiiigs added out of the Obfer-
vacioas of the aiacient Writers of Natural
Philofophy and Natural Hiñory, where they
fccmed furthcr to expíala and illuftrate Mat-
íers, ín all which, toayoid RepetitionjGrati-
íude demands that the Readcr fhould know
that thcre are a great many Things owing to
íhclearnedand induflrious Dr. Laughton,^ná
to the Reverend Mr. Morgan. The former
of which communicated a great many Things
dirperfed throughout the whole Book, and
correcfcd A-bundance of Errours : And fix
whole Diílertations are owing to the latter,
■viz. Thol’e concerning the Laws of com-
municatíng Motion in elaftick Bodies ; The
Explication of the Forces of the mechantck
Towers, which are contained in this firft
Part : and thoíe concerning the Celerity with
'which heavy Bodies defeend } the Motion
of ’Troj entiles j the Motion ofBenduiums j
and that concerning the Rainbow j which
are contained in the following Parts.

The foLirth Part of this Work is but
íhort, and not very perfedl: j wherefore it is
thought more advifeable to refer the Read-
er to later Writers of Anatomy who have
handled that Subjed clearly and fully, than
to tranferibe fo many Particulars. I hope
the Whole wiii be agreeable and acceptable.

THE

Ailthor’s PREFACE.

HE Treatifcs of Natural Phi-
lofophy which have hitíierto
bccn publiíhed, bcing prctty
much alike, both as to the
Matter of them, and the
Manncr of handiing them ;
It is eafy for me to forefee, that amongíl
thofe who read This, there will be a great
many who will be at firñ furprifed at the
great Differcnce there is between this Trea-
tife and others. To prevent therefore in
fome Meafure this Surprize, and to give
what Satisfadlion I can in this Matter, I
think my felf obliged to give an Account
of the Obfervations which I have made up- -
on the Philofophy of the Ancients, and of
the Method which I have taken in this
Work.

In refleéfing upon the different EíFeds of
Time, I have long fmee obferved, how fa-
Vourable it is to fome Things, which it
is continually advancing to Perfeftion, and
how pernici^s it is to others, fo as to
fttip x\i.zíV!.^í^ thofe Beauties and Graces
A 3 which

The Author’s T RE F AC E.

-which they had at their firft Rife ; and I al-
ways concluded that Arts and Sciences can-
not be of the Number of thefe latter, but
that Time is fo fat from being prejudicial to
thera, that on the othcr Hand it is very ad-
vantagious. For as a great Number of Per-
fons who cultivare the fame Art or Science
for feveral fucceeding Ages, add their own.
Induftry, and their new Light to the anci-
ent Difeoveries of thofe who went before
them, it is impofllble but that fuch an Art
or Science muft receive great Improvement,
and arrive nearcr and nearer to its utmoft
Perfedion.

And thus I faw that Mathematicks did
really increafe by little and lirrle in this
Manner | as it is eafy for any one to be con-
vinced of, who confiders only the vaft Pro-
grefs that hath been made by the great Ge-
nius's of our Time, wdio have excelled all
others in this Particular, and furmounted
fuch Difficulties as the moft Learned in for-
jner Ages confeíled they were not able to
folve. I faw alfo that moft Arts were per-
feded by Time 5 Workmen every Day find-
ing ouc a Multitude of curious ínventions,
which are not fo much cfteemed as they
deferve, becaufe they are very common,
and we do not enough take Nptipe of
them. Though amongft thofe Engines which
are craploy.cd in making Thingsof common
Ufe, there is One that has been lately in-
vented, which has in it fo.^ much Contri-
vanee, that this ítngle Thin^:’cferves to be

more

The A u't h ó r’s T RE F A C E.

more admired than all the Inventions of
Antiquity. '

But when I carne to confider Philofo-
phy, particularly Natural Philofophy, I was
very mucli. furprized to fee it fo barren as
not to have produced any Fruit j in fo much
that twenty Ages have paíTed, without any
new Difeo very made in it.

How.bver, i could not perfuade my
felf, that the Study of Natural Things was
negleíted, becaufe it was thought to be of
no Ufé; for Health has always been efteem-
ed one of the chief Bleílings of Life, and
no one can be ignorant, that Phyííck, the
fole End of Which is to maintain and re-
ftore Health, is built upon Natpral Philo-
fophy.

• Ñor could I ever perfuade my felf, that
thofe who improved this Science .were lefs
ingenious, than common Artifts..: For we
find by Experience that in Families Jwhere
there are a great many Childreri, when
they come to make choice of their/Profef-
fions, thofe of them which have the quáclc-
eft Genius, are appointed for Study, or vo-
luntarily indine themfelves to it j and thofe
only whofe Underftanding is not fo good,
apply themfelves ■ to the mechanicál Arts,
and are contented wirh their Lot.

Heréupon i fufpedted, that perhaps
the Knowledge of Natural Things was a-
bove the Reach of humane Underftanding,
fo that it was in vain to labour to attain that
our Capacity : but whe-n/I
A 4 confidefd

which is be,al)nd

f/

l^he Author’s T REFACE.

confidered the furprizing Things done by
fome Philqfophers of our own Age, who
withinforty or fifty Years have found out
Things which were looked upon as moñ dif-
ficLilt, and which fome have doubtedy whe-
ther evcr they could be found out at all %
1 immediately caftoff this Sufpicion.

So that 1 was forced to conclude, that
the: Manner of philofophizing, was thc
Thing that had hitherto been miftaken, and
that theTrrors therein which have been intro-
duccd, .'being fuch as no Body had any Hopes
of finding out a Remedy equal to, were
a certain Bar to hinder the Approaches to-
wards Truth. 1 fet my felf then to enquire
w.hereih.Thfi Manner of their treating Phi-
iofophy was defe^tive ; and after having ex-
arained . with the grcatcíi: Diligence polliblc*,
what the Method has been from the Schools
of Atheniam down to this very Time ;
there feemed to me tq be four Things blame-
able in this Matter.

Eifji, The too great Authoriry that hath
always been given to the Ancients in the
Schools Tor befides that this prodigious
Diíference which is put between them and
the Moderns, is without the leaft Foundati-
pn 5 (for Reafon is to be found in every
Place andrcvcry Age 5) it is .eertain that fuch
a blind Submifllon to the Opinions of An-
tiquky^ is .the Caufe why Perfons of the
greatcfc Genius, (receiving fuch Opinions for
truc without coníidering them, when per-
haps they may be falfe,) have

T!he Author’s T RE F A C E.

portunity of knowing the contrary Opini-
ons, ñor confequently of finding out all
thofe other Truths that depend upon thofe
which fo fatal a Prejudice has hindred them
from feeing. And further, this ftrong Per-
fuaííon of our being fo much inferiour to
the Ancients, caufes in us a Kind of Slug-
gifhnefs and DiíEdcnce, which hinders us
from attempting to enquire into any Thing
at all. We imagine that Reafon is limited
at the Place where they flopped, and that
all is done th'at can be done humanely/
fpeaking, if we go as far as they went.
Thus the greateft Genius’s contenting them-
felves with going over the Rcafonings of the
Ancients, don’t exercife their own Reafon
at all 5 and though they be never fo capablc
of finding out any Thing themfclvcs, they
contribute no more to the advancing Na-
tural Philofophy, than if they had not med^
dlcd with it at alL

I íay nothing particular of that Venc-
ration which hath been paid to Ariftotle,
though fometinies it has rifen to fuch an Ex^-
qefs } that to alkdge that he faid fuch a Thing,
was fuíücient to make any One not only
to doubt of what his Reafon convinced him,
but even to condemn it, I fhall only mak,q
this Obfervation ; that the jmagination
which a great many have had, that He kriew
all that could be known ; and that all Sci-
ence was contained in his Bocks, hath
caufed the greateft Parí of the beft Philofo-
phcrs lince tqj^ply themfclvcs in vain to

The A u T H o r’s T RE F A C El

read his Works, to find out in them what
was not there, and what they might perhaps
otherwife have found out by their own Ini
genuity. But if there have been fome
who, not being quite fo zealous as others,
did not hope to reap fo very much Fruit
from reading him j yet it always happened
that the Defire of recommendlng themfelves
by explaining thofe Places which he left
obfcure (on purpofe, as fome think, or elfe
for want of better Light) hath made them
employ their whole Strengtfi of Mind,.and
all their leifure Time, to very little Purpofe,
in wricing Comments upon his Philofophy,
without promoting the Science at all : For
thofe who have undertook to explainTfrz-
Jiotle ; have underftood hini fo differently,
that there are an infinite Number of Places
which all the Schools are divid ed about-;
And if there be fome few in which they
have agreed, it is becaufe the Notions con-
tained in them were fo common, that ve-
ry few Perfons were ignorant of them. . So
that they took more Pains to ftudy Arifiotle
than they did to Üudy Nature, which per-
haps is not ncar fo mytterious as He. There
are a MuLtitude of Things which Nature
plainly declares to thofe who apply their
Aíind thercto. But alas, this is not the Cu-
ítom, we had rathcr hearken to Arifiotle and
íhc Ancicnts ; and this is the Reafon why
we make fo little Progrefs.

Ano TH Eli Thing which hinders the
Progreís of Natural Philoífediy, is theTreat-

The Author’s T REF AC E.

ing thereof in a Manner too metaphyíícal ;
and the Difputing about Qneftions ib abflraft
and general, that though ail Philoíophers
were agreed in their Notions of theni,
yet they would not help to explain the
leaft particular Effe£t in Nature; whereas
cvery ufeful Science ought to defcend im-
mediately to Particulars. Por inftance, wliat
good do thofe long and nice Difputes do,
aboiít the E>i’ütfibility of Matter ? Por
though it could not be accurately determi-
ned, whether it be infinitely diviíible or
no 5 it would be fuííicient to know, that
it can be divided into Parts fmall enough
to íerve for all Purpofes that can be.

I T is very ufeful, wiíhout doubt, to find
out the Nature of Motion in general. And
it may not be very improper to examine a
little whether it be well or ill defíned thus,
The AA of a Being in Tower, fo far forth
as it is inRower. But weínould not fpend
too much Time in determining ,this, and
fuch like Qucftions ; 1 íhould rather think,
that after-having confidered a little the true
Nature of Motion in general, we fhould
particularly and diflinéily examine all the
Properties of it, fo that what we affirm
concerning it, may be applied to fome
Ufes In a Word, 1 thiuk ,we íhould careful-
ly enquire into the Caufe why Matter pro-
duces fuch a particular PíFeft rather thán
any other, and not accuñom ourfelves to fay
that it is the Pff^ of a ceitain fiuality ; for
from henee that we are led to give

Words

í/

Thi, Á u T H o r’s T re F a C E.

Words inftead of Reafons, and henee arifes
that fenfekfs Vanity of thinking that wc
know more than others, becaufe \ve know
Words which the common People don’t
know, and which indeed have no deter-
minare Meaning. To fay the Truth; it
íhows a mean Spitit, and one that is foon
facisfied ; to believe that we know more of
Nature than other Men, becaufe we have
jearn'd that there are occult Qualities, and
can give a General Anfwer to all QLieílions
propoíed to us concerning the diíFercnt
EfFeds of Nature. For what DiíFerence is
there in the Anfwer of a Plowman and a
Philofopher, if they are both asked,
whence is it, for Inflan ce, that the Load-
ñone attrads the Iron, and the one anfwers,
that he does not know the Reafon of it, and
the other fays, it is done by fome Vertue or
occult Qtiality í Is not this in plain EngUpij
to fay the fame'Thing in diíFerent Words? •
and is it not evidenr, that all the Difrerence
there is betwixt them is only this, that the
one is fo honeít as to confefs his Ignorance, '
and the other has the Vanity to endeavone
to conceai his ?

A third Defeft which I have found in the
Method of Philofophcrs, is, that lome of
them are wholly for Rcafoning, and depend
fo much upon the Strength of their Argu-
ments (efpecially if they be borrowed from
the Ancients) that they judge it fuperfiaous to
make any Experiments. _ Others on the
C:ontrg,ty, quite tired vith'Vfch tedious Ar-

guate tus,

The A u T H ó r’s T RÉ F ace.

guments, the greateft Part of which are not
concluííve, or áre nothing to the Purpofe,
think evcry Thing ought to be reduced to
Experiment, and that thcre íhould be no Rea-
foning at all. But boththefe Extremes do c-
qually hinder the Progrcfs of Natural PM"
lófophy. For they who fall into the firft
of thefe Errors, hinder themfelves of the
beft Means of fínding out new DifcoverieS;,
and of confirming their own Arguments
likewife s And they who fall into the fe-
cond, by depriving themfelves of the Li-
berty of drawing Conclufions, hinder the
Knowledge of a large Train of Trutlw,
which may many Times be deduced from
one ftngle Experiment. Wherefore it can-
not but be very advantagious to mix Expe-
riments and Arguments together. For Rea-
foning perpetuaihy, and upon fuch general
Things only as are ordinarily argued about,
wichoLit defcending to Particulars, is by no
Means the Way to attain any very cxten-
ííve or very certain Knowledge : Thus we
fee the fame Things continually bandyed
about, and no new Difeoveries made 5 nay,
we are not very fure of the oíd ones, as
general as they are. We fee alfo that they
who confíde moft inthofc Arguments which
they believe to be Arijlotles, are in perpe-
tual Dirpute,.and that they contend for Opi-
nions which are direftly contrary to one
ánother, without being able to convince
thofe of the other Side by their Arguments.
And this plaiA^ íhows how littie Certain-

The Authors T REF ACE.

ty; or Evidence there is in their bare Reafon-
ing.

Experiments therefore are neceíTary
to eftabliíh Natural Philofophy 5 and this
was a Thing which Arifiotle was fo fully
convinced of, that the Reafon why he
thought that vcry young Perfons fhould not
apply thcmfclves to the Study of Natural
Philofophy, was, becaufe at that Age they
are fo llttle acquainted with Things, as to
be unable to have made many Experi-
meiits and on the other Hand he was of
Opinión, that they were then moft capable
of receiving Mathematicks, becaufc this
Science confifts of ineer Reafoning, of which
the Mind of Man is naturally capable, and
docs not at all depend upon Experiments.

B u X on the contrary to rejed entirely all
Reafoning, in order to do nothing but
inake Experiments, is to run into another
Extrcmity much more prejudicial than the
formcr. For this is wholly to difcard Rea-
fon, and yield all up to Senfe, and to con-
trad our Knowledge into a very narrow
Compafs i for by Experiments we can come
to the Knowledge of grofs and fenílble
Things only. Whercfore if wewouldpro-
cccd rightly in our Enquiries into natural
Things, we muft of Neceílity mix thefe tw.o
Mcans of Knowledge together, and join
Reafon with Experiments.

And that we may the better fee the good
Effeds of thefe two whcn joined together,
and the Ufe that may be — them.

to

The A u T H o r’s T re F A C E.

to the Advantáge of Natural Philofophy,
we may obferve that there are three Sorts
of Experiments. The fírft is, to fpeak pro-
perly, only the mere limpie uíing our Senfes }
as when accidentally and without Delign,
cafting our Eyes upon the Things around
US, we cannot help taking Notice of them,
without thinking of applying what we fee
to any Ufe. The fecond Sort is, when we
deliberately and defignedly malee Tryal of
any Thing, without knowing or forefeeing
what will come to pafs 5 As when, after the
Manner of Chymifts, we make Choice of
firft one Subjeftand then another, and make
al-1. the Tryals we can think of upon each of
them,; and carefully remember what we
have , at any Time found to fucceed, and
the Manner in which we arrived at any
certain. Effeft, .in Order to apply the fame
Means another Time to produce the fame
Eífeíf. We alfo make Experiments in this
fecond Way, when we go amongfl; diffe-
rent Sorts of Workmen in Order to find
out the Myfteries of their Arts, as Glafs-
makers, Enaraellers, Dyers, Goldhniths, and
fuch as Work dífferent Sorts of Metals, and
to obferve how they prepare t¿acir Materials,
and how every one of them afterwards work
upon thofe which belong to them. Laftly,
The third Sort of Experiments are thofe
which are made in Confequence of fome Rea-
Joning in order to difeover whether it was juft
or not. As when after having coníidered the
..I ordinary

The Authors T RE F A C E.

ordinary Effedls of any particular Subjed, ,
and formed a truc Idea of the Nature of
ir, that is, of That in it 'which makes it
curable of producing thofe Efebís } w©
come to know by our Reafoning, that if
■what \ve believe concerning the Nature of
it be true, it muft ncccflarily be, that by
difpoíing itafter a certain Manner, a new
EfFe£t will be produced, which we did not
bcforc think of j and in Order to fee if this
Reafoning holds good, we difpofe the Sub-
je£tin fuch a manner as we believe it ought
to be difpofed in Order to produce fuch an
Eífed.

Now it is very evident that this third
Sort of Experiments is of peculiar Ufe to
Philofophers, becaufe it difcovers to them.
the Truthor Falfiry of the Opinions which
they llave conceived. And as to the two
forcgoing ones, thoug’h they be not altoge-
ther fo excellent, yet they ought not to be
wholly rcjeded as of no Ufe to Natural
Philorophers : For bcfides that their Know-
ledgc iscontinually cnlarged by them, they
are alfo the Occafion of making the firft
Con) edures concerning the Natune oX thofe
Subjects’ which éíatural Philofophers are con-
vcrfant about ; and , preferve them from
fome falfe Notions they might otherwife
perhaps have entertained. Thus, for In-
fíancc, we might have concluded in gene-
ral, úiZtCold contra £ís and cóndenfes every
Thing, if wehad not difcoveredby Chance or

The Authors T RE F A C E.

otherwife, that there are Things which are
dilared by Coid.

The fourth Defedl: that I obferved in the
Method of PhilofopherSjis the negleding Ma-
thematicks to that Degrce, that the very firft
Elemcnts thereof are not fo much as taught
in theiü Schools. And yet, which I very much
wonder at, in the Diviíion which they make
of a Body of Philofophy, they never fail to
make Mathematicks. one Part of it.

N o w this Part of Philofophy is perhaps
the moft ufeful of all others, at leaft it is ca-
padle of being apply’d more Ways than all the
* others : Por befides that Mathematicks teach
US a very great Number of Truths which may
be of great Ufe to thofe who know how
to apply them ; They have this further very
conííderable Acivantage, that by exercifing
the Mind in a Multitude of Demonftrati-
bns, they form it by Degrees and accuftom
it to difeern Truth from Falfehood infinitely
betrer, than all the Precepts of Logick with-
.out Ufe can do. And thus they who ñu-
dy Mathematicks find themfelves perpetu-
ally convinced by fuch A.rguments as it is
impoffible to reííft, and learn infenfibly to
know Truth and to yield to Reafon 5 info-
much that if inftead of neglefting them, as
is ufually done ; it were an eflabliíhed Cuf-
tom, to make Childrem apply themfelves to
this Science at firft, and to improve them
in thefe Studies as much as wc do in others ;
it would be of vaft Ufe to hinder them
from contra¿lj,;^l| that invincible Obftinacy
; . ^ ■' b in

/

The: Author’s T RE FACE.

in their Opiiiions which \ve fee in the great-
eft Part of thofe who have compkated their
Courfe of Philofophy 5 who probabJy would
not have fallen into fo pernicious a Tem-
pcE of Mind, if they had beeii accuftomed
to, and familiar with convincingTruths ; and
not feen thofe who maintain in publick any
Doftrine whateyer, continually triumph
over thofe who endeavour to íiipport the
contrary ; fo that all Things feem to them
oniy mere Probabilities. They do not look
upon ftudying as a Means to difcover new
TrnthS;,. but only as a Picce of Wit to ex-
ercife themfelves in, the only End of which
is fo to confound Truth with Falfehood by
Means of fome fubtle Diñinftions, that the
one ór the other may be equally maintain-
ed, without ever being cotnpellcd by any
Reafons to yield, let the Opinión they de-
fend be never fo extravagant. And indeed
this is the Event of all publick ' Difputes,
where very often Opinions diredly contra-
ry ta each other, are by Turns propofed
from the fame Chair, and equally triumph-
cd in, without raaking Matters at all clearj
or eftablifhmg any Truth thereby.

But the great Advantage that natural
Philofophers . have from Mathematicks in
particular, is, that they are thereby accuft-
omed to the viewing of Figures, and ena-
bled to underftand the diíFerent Properties
of theni. I know it is here objefted by
fome, that we ought not to ñop at Figures,
becaufe they are not But though

they

The Author’s T RE F A C E.

they are not a£íive in themfelves, yet it is
certain notwithftanding that their ’T>tffer~
enees make Bodies, which \ve put into Adi-
on, capable of certain EíFeds, which other-
wife they could not ha ve produced. Thus
a Knife by having an Edge íet upon it be-
comes capable of cutting, which before it
was not -y and Workmens Tools, by their
diíFerent Figures, are fitted to produce thofe
difFerent Work,s which are made by the
Help of them. And if the Figures of Bo-
dics, which come under our Senfes are fo
neceíTary to the EíFeds which they produce,
it is reafonable; to think that the moft im-
perceptible Parts of Marter, feeing they háve
every one a certain Figure, are alio capable of
producing certain Effeds in Proportion to
their Bignefs, like thofe which we fee pro-
duced by the groíTeft Bodies.

B u T not to enter too far into Particu-
lars concerning the great Ufe of Mathe-
maticks, Is it not enough to put us upon
applying ourfelves more to them than we
have hitherto done, to conííder that 'tis by
their Means that the modeni Philofophcrs
have difeovered all that is excellent and pecu-
liar in natural Philofophyí And alfo thatit is
by the Help of Mathematicks, that the moft
celebrated Artifts in every Age have rnade
all thofe noble Difeoveries, the Ufe .of
which is fo advantagious to us at this very
Time, and which make all the Variety of
Arts and all the Convenienccs of Life. It
may be íome^, ;lay think, on the contrary,

b 2 that

The Author’s ^REFACE!

that thefe very Artiñs, the greateft Páit of
wbom it is very probable have n'ot muclv
ápplied themfelvcs to this Science, will juf-
tifie it, that it is not fo neceíTary as I would
perfuade them. But here there are two
Things to be coníidered-; Firíl, that as there
is a natural Logick in all Men, fo is there
alfo natural Mathematicks, which accord-
ing as their Genius's are dilpofed, make them
more or lefs capable of InventioUí Se-
condly, That if their Genius alone, con-
du£ted only by natural Light, will cariy
them fo far, we can not but hope greater
Things from the fame Genius if the Study
of Mathematicks be added to its natural
Light, than if that Study be ncgledted. And
indeed all the Propolitions in Mathematicks,
are only fo many Truths, which thofe,
who apply themfelves to them, come to
the Knowledge of by good Senfe. And
they who find themfelvcs naturally difpofcd
to it, do very ill to negled' what others
have before difcovered : Por it is the moft cer-
tainWayof findingoutany Thingnew, to
know all that has been before found out by
others, and the Manner how it was found out.

However, i don’t put them upon
the Rank of Inyentors who have met
with fomething by Chance which they did
not fearch after ; As was the Cafe of that
Workmanwho by cooling on a fudden in
the W atcr a Piece of Steel which he had
heated red hot, found it in a Moment ve-
í-y much harder than it waioefore ; It ,was

without

The Author’s T RE FACE.

wkhout doubt a very lucky Thing to find
out this Way of tempering Steel 5 but the
Workman who liad the good Fortune to
hit upon it, does iiot deferve the Ñame or
Title of. an Inventor i as a great many o-
thers do who are not beholden to Chance
for the Glory of their Inventions ; As for
Inflance, the Perfon who firñ invented a
Fire-lock to a Gun ; for it is certain that
this latter had the whole Engine in his Head,
if I may fo fpeak, before he made the leaft
Part of it, whereas the other found out the
Way of tempering Steel, by hitting upon
a Thing, as was before faid, by Chance,
which he did not fearch after.

Lajlly, That Mathematicks are of ve-
ry great Ufe in the other Parts of Philo-
fophy, we need no other Teftimony than
that of the moft celebrated ancient Philo-
fophers, who not oiily fpeak honourably
of them in their Writings, but do alio
malee ule of them themfelves. It is fufficiently
known, that Flato caufed it to be written
over his School Door, That none but Geo-
metricians fh^uld enter in there. And they
who have taken the Pains to read over the
Works of Ariftotle', have taken Notice o£
the feveral Applications he has made of Ma-
thematicks in many Places j fo that they
who do not underftand the Elements at
.leaft, have no great Reaíonto boaftof their
being able to underftand the Writings of
this Philofophe^

' b 3 The

/ ,

The Author’s T REFACE.

The more I coníídcr thefe four Dcfeds
in the Method of Philofophers, the moreí
find it impoílible to come to the Know-
ledge oí philofophical Truths, without cor-
reftingthem. And this does not appear to
me to be very difficult ; for though I had
made fome Próficiency in Mathematidks,
and accuftomed my felf to follow Reafon
rather than Authority, yet I did not find
my felf fuch a Lover of my own Reafon-
ings, as to negleft Experinients, ñor fo bent
npon Experiments, as not to fuffer my Rea-
fon to -go beyond ^what they difcovered.

But though fhis was fufficient toputme
upon impro ving natural Philofophy, and to
malee me hope that 1 might be able in fome
Meafure to help forward the Progrefs of this
Science 5 yet 1 obferved a fifth DefefV, not
in the Method of thofe who ftudy Philo-
fophy, but in thut of a great many who
rcad their Works; which made me think,
that to publiíh any Thing upon natural Phi-
lofophy, was fo far fróm being any Advan-
tage, that it Was But too much to expofe
one’s felf, For that Averfion which is ufu-
al againfl: fuch Perfons, and that difagree-
able Manner in which thofe who are um
capable offindingoutany Thing theirTelves,
receive the Wrkings of fuch as attempt to
excced what is common, often hazard the
Reputation of the Author. For fcarce can
arPhüofopher prefe'nt the Publick with any
Fruits of his Studies, but-ífome unknown
Perfon who has a Mind to^gnalize him-

felf.

The Kv rno-SLS T R‘E FAC E.

felf, attacks them before he underflands them.'
And henee come thofe trifling Difeourfes
or DiíTertations, for the moft Part anony-
moLis ones, which never fail immediately
to appear, wherein are feldom any Thing
elfe but Reproaches and very low Jeñs j
and not being able to overthrow Trurhs
that are fo firnily cñabliíhed 5 they try to
turn them into Ridicule, by íliowing that
they are contrary to Tome ancient Maxim
or popular Error, which tickle the Ears of
half-witted People, who are accuftomed to
take Things without any Proof : And that
which is very remarkable here, is, that thefe
Writers for the moft Part attack the Works
of others only bccaufe they think them con-
trary to Ariftotle ; and yet beeaufe they have
rcad nothing of this Philofopher but only
thofe Citations which they found in their
phílofophical Leftures, it very often hap-
pens that the, Thing which the>\ thus at-
tempt to confute, is what Ariftotle himfelf
has íaid in exprefs Terms. We may fafely
affirm, that the Ancients did more Juftice to
Men’s Labours, and without doubt it was
in a good Meafure owing to this, that Phi-
lofophy made fome Progrefs in the firft A-
ges of it ; fo far were they from fuíFering
thofe who had made any new Difeoveries,
to be cried down at a Venture and without
any Rcafon ; every Body knows that there
were publick Rewards appointcd for fuch 5
even to have f^metimes Sratues ereded to
them j fo h^ly were they perfuaded in
b 4 thofe

The Author's T RE F AC E.

thofe Times that Honour contributed moft
to the Invention of Arts.

I T is truc indced, that this Maxim feems
to be revived and re-eftablifhed in our Age.
Yct though Princes liave by theit Authori-
ty approved and favoured Arts and Scien-
ces, the long StiíFnefs which they who
ñudied natural Philofophy have in fo ma-
ny Ages contraded, have fo accuñomcd
them to reft íatisfied 'vith what they recei-
ved from their Prc ..eceíTors, that the very
propoíing any new Thing, is cnough to ten-
der both the Thing and him that propoícs it
odious. Now to take away the Foundati-
on or rathcr the Pretence of this Averíion,
fuch PeiTons ought to know, that this Re-
proach of Novelty is generally a great De-
eeit : For if a Thing be true, it cannot be
new, becaufe nothing is fo ancient as Truth,
and it is the Difcovery of the oppofite Er-
roLir only that can be faid to be new, For
Want of rightiy diílinguiüiing thefe two
Things, we often fee fome Perfons crying
out that we overthrow the Order of Na-
ture, whcn we only overthrow a falfe O-
pinion which they \v ere prejudiccd in. But
though fuch Sort of Perfons have not much
Reafon on their Sidc, yet the Credit and
Authority which they may have over others,
is the Caufe of their Exclaraations always
making an Imprcíiion upon the Minds of
a great many ; and this muft ever be difa-
grecabie to thofe who hav^ no other De-

The Author’s T RE F A C E.

íísn, but to contribute to the Publick
Good.

W H A T a Vexation muft it be to Dr. Har-
'uey, for Inftance, to fee ali his Life long,
how ili the Difcovery he had made of the
Circulation of the Blood, was received ;
the Motion of which was quite diffcrent
from what the Ancients thought ? Surely
we cannot fhow toogreat an Acknowledg-
ment to a Man who had undcceived the
World of an ancient Errour, and by the
Truth which he cftablifhed, made us fee as
clear as the Day, that almoft all the Theo-
ry of the Phyíick of the Ancients was falfe.
But how many Enemics has this Dodtrine
got him inftead ofThanks? I folemnly de-
clare therefore, that upon feeing what Li-
berty is taken to oppofe the bcft Things,
becaufe the Misfortune of Mens having al-
ways been ignorant of thera, made them to
be thought new i 1 laid aíide the Thoughts
of ever entertaining the Publick with any
Thing of my own, or what I learned from
the Works of fome modern Writers. But
thus much I thought at leafl, that it was
not impofllble to advance a little further
than is generally done in the Knowledge of
Natural Things, if I carefully avoided fal-
ling into any of thofe Defefts which I ob-
ferved in the Method this Study was in at
prefcnt. And indecd having fpent fome
Years in reading the Ancients and Moderns,
but with a Refolution not to follow
them any furtk2r than I could fee the Rea-

fons

The A u T H o R s T RE F AC E.

fons of each of them 5 it appeared to^ine
that my Defign was not entirely fruflrated.
But while I was thus inftrafting my felf by
reading Books, and converííng with learn-
ed Men, and thoíe that wer-e excellent ín any
Art, I never laid aííde the Ufe of my Rea-
fon, but coníídered the fcveral Subjeds, and
endeavoured always to ground my Reafons
itpon mathematical Truths, and fure Expe-
riraents. And fo good Succefs had 1 in
carrying on my Defign, that a great many
pf my Fricnds, whofe Abilkies all the World,
I faw, had a great Valué for, advifed me
to communicate it to others by publick Con-
ferenccs, ór atleafl by prívate Converfation.
I muft fay, that it was very difficult to per-
fuade my felf to this, becaufe I am diftruft-
ful of my felf, and do not think my felf
Oratour good enough to undertake to picad
the Caufe of Truth thus publickly. However
1 fuíFercd my felf to be over-ruled ; and
though I was feníible 1 wantcd a great ma-
ny Talents, yct I fubmitted to my Fricnds,
who aífured me, that if the Things wcre plain-
ly propofed, and in a mathematical Way,
they would be acceptable at leañ to the beft
Judgcs, And indeed their Advice fuccecd-
cd : For thefe Conferences wcre not only
agreeablc, but it wa? wiíhed that the Sub-
jeds had been put down in Writing. And
by confenting to this Opinión of my
Fricnds, 1 perceived that 1 had infenfibly
wrote- a Book 5 and bec ’

many Copies of it aboui

The Author’s T REF AC E.

come, as it were, publick, and a greát many
Faults ílipp’d in, I refolved to rcview it
more exadly, in Order to perfed it as much
as I could. They who rcad it ovcr, wiil
eafily fee, that I have overlooked nothing
that is good in the Anclen ts.

I have taken ali the general Notions
from Ariflotle, eithcr for the eftabliíhing
the Principies of natural Things, or the
chief Properties of them : And I have re-
je£ted a Vacuum and Atoms, or Epicunis’s
indi'vijible TarticleS:, which 1 think are
Things contrary to what is fírmly eílablifh-
ed by Arijiotle ¡ and I have learnt of him
to coníider with the greateft poílible Care
the different Bignefles, Figures, and Moti-
ons of the infenfible Parts of which
fenfible Things are compofed. And this I
was the readier to do, becaufe all thefe
Things have a neccíTary Connexion with,
and Relation to the ‘Divijibility of Mat-
ter, which I acknowledge with Arijiotley
who hardly reTolves any particular Qucfli-
on, without coníídering the Bigneís, Fi-
gure, and Motion of the Parts of Bodies,
and the Porcs which are between them.
But that which moft of all determined me
to this Coníideration, was, that though
there feems to me to be a juft
Ground to doubt of the Truth of fome
Gfialitíes and Fowers commonly afcribed
to fome Bodies, yet I do not think that
there is the fan^f Reafon to doubt of their
being compofí'd of infenjible Tarts, or

The Author's T RE F A C E.

that I can be deceived in affirming that all
thefe Parts have their particular Figure and
Bignefs.

B E s I D E s thofe Aíliílances which I had
froni the Ancients, I have alfo colleded a
great many other Truthsj from the moft
eminent modcrn Philofophers, whofe Ñames
you may find in their Places. But the Per-
fon whom 1 have moft of all made Ufe of
in this Work, and whofe Ñame I have not
mentioned at all, to avoid perpetual Repc-
tition, is the famous Caries 5 whofe Merir,
by which he becomes more and more known
to all the Nations in Euroj)e, as he has
long been to many of the principal States,
will draw a Confeílion from the whole
World, that France ís at leaft as happy in
producing and educating great Men in all
Sorts of Profeñions, as ancient Greece was.

I have divided this W ork hito four Parts,
The firft treats of natural Bodies in general,
and their principal Properties, fuch as ‘Di-
vifibility, Motion and Re [i, of Elements,
and of fenfible G^iialities, and I liave particu-
larly infiftcd upon cxplaining thofe which re-
late to Seeing. And I flatter my felf that
upon this ftngle Subjecl I have collefted
more Truths into eight or nine Chapters than
are contained in feveral large Volumes
which treat ofOpticks, Dioptricks and Ca-
tropticks after the Manner of the Anci-
'ents.

The fccond, treats of fjpe Syftem of the
JVorld, or of Cofmography, i^ch I thought

might

The Author's T REF ACE.

might prove more ufeful than tile general
Queftions that are ufually propofed in the
common Books of natural Philofophy, which
are as it were Commentaries xx^onAri/loíles
Books concerning the World. I have alio
treated of the Nature of the Stars and
their Influences. And after having explain-
ed wherein Gravity and Levity coníift
(which 1 could not fpeak of in the firft Part,
not having premifed what was neceíTary,) I
eonclude this Part with explaining the Flux
and Reflux of the Sea.

The third Part is takeii’ up in explain-
^ ing the Nature of the Earth and of ter-
reflrial Bodies, that is, of the Bodies con-
tained in ity or which furround it, as Air,
Water, Pire, Salís, Oyls, Metals, Mine-'
rals, and Meíeors.

Laftly, I have endeavoured in the fourtii
Part to comprife all that is hitherto, with
any Certainty, known of the Animal
Body.

One Thing perhaps will be obferved in
the Method I have taken, viz. that I have
been pretty long and particular, in explain-
ing, in the firft Part of this Book, all the feníi-
ble Qualities, which Philofophers ufually
explain, and that but briefly, at the End of
their Treatifcs of Philofophy, in which thcy
comment upon thefe Books of Ariftotle's
concerning the Soul. The Reafon of which
is, becaufe this tcaches us to know ourfclves,
and becaufe hcreby we are feafonably freed
from a popu';.'? Errour, and a Prejudice

which

The Author’s T REFACE.

•which we have entertained from our In-
fancy, which 1 have known by Experience
a great many never to have bcen able to get
ridof, not even after they have gone through
their whole Courfe of Ledures, but have
brought back from the Schools thofe Ha-
bits they earried thither, viz. the afcribing
their ówn Senfations to the Objeds which
caufe them, and the coníidering theíe Seii-
fations as Qnalities in the Objeds.

Further, yon will not find a great
many Things in this whole Trcatifc con-
trary to Arijlotle ; but you will find more
than I could wiíh that are contrary to moft
of the Coramentators upon hira : And be-
íides this, you will meet with a great ma-
ny Things, which nekher Arijlotle ñor his
Followers have treated of at all, which I
have however judged more ufeful than ma-
ny others which Philoíophers have wholly
imployed themlelves in. And in all this í
did not think it very ill in me to depart
from fpme particular Notions, when I
found that thcfe Notions were difagreeablc
to Trnth.

But what has very rauch abated thofe
Scruplcs which I had about this Matter, is,
rhat when I carne to compare thofe Places in
^hisTrcatife which are contrary to Arifto-
tléy with the, Writings of the publick Pro-
fcílbrs of his Philoíbphy, I could not find near
fo many in my own W orks as in the Works
of others. And without enumcrating the
Particulars, it is eafv to bC (atisfied herein,

if

The Author’s T RE F AC El

if we do but conííder, that there is fcarce
any Queftion in Controverfy, but one half
of them draw Concluííons diredly contra-
ry to the other half. Whence it follows,
that we muft neceíTarily find in the Writings
pf thofe who profefs to teach the Dodrines
of Arijiotle, as many Places agafñft him as
fot him.

But though all the Philofophers did a-
gree with each other and with AriJiotkj I
don't fee that this Agreement of ' theirs
OLight to forcé me to be of their Opinions,
ñor that Philofophers can prctend that I
am obliged to follow them, in what I ani
fully perÍLiadcd and convinced they are in
the wrong of. Por lince it is the Cuftom
with them to propofe the Matrers which they
treat of, in the Form of Queftions, this
Véry doubting Manncr of theirs íhows that
there is a perfed Liberty oí taking that 5ide
which we think to be nioft reafonable. In
what Manner my good Intentions will be
received Time will fhow. However, I am
preparihg a Latín Verlion for the Ufe of
Foreigners, with whom I hope to meet
with a favourable Reception.

^ ' ‘■ 41 ’ %

THE

CENTS

o F

P A R T I.

C H A P I . The Meaning of the Word Ploy/tch^
and thé Manner of treating fuch d SubjeSi.

2. Añ Examination of the Notions that pre-
cede-.the SMidy of Natural Philpfophj.

3. Phe Manner 'of applying Phüofophy to parti-
cular SubjeSis.

4. A Caution. concerning Words.

5. Phe principal Axioms of Natural Philofophy,

6. Of the Principies of Natural Phings.

7.' Of Matter.

8, Sofne CoroUaries.of the foregoing Notion.

.9. Of the Divifibility of Matter. -
10. Of Motio.n and Reji.

II. Of the Continuation and Cejfation of Motion.
12 . Of fuch Motions as are cofnmonly afcribedio the
Fear of a Ysícunm.

13* Of the Oetermination of Motion.

Of the Compofition of Motion, and ofits Petera
mjnatmt.

T 5- Of Reflexión and RefraPlion.

16. Of hará Bodies put inío Liq^uors.

17. Of Accretion, Dmhnition, and Alteration.

18. Of Fornis.

19. Of pletnents aecording to the Opinión of the An-
cientSi ■

20. Of the Elements of tN Chyrnifts.

21. Of

The C O N T T N T S.

21. Of the Elements of natural 'Things.

22. Of the Farm of a Hará and of a Liquid Bodj,
or of Hardnefs and Liquidity.

23. Of Heat and Coid,

2¿f, Of Faftes.

25. Of Smells.

26. Of Sound.

27. Of Light and Colours, and of Franf^arency,
and Opakenefs.

28. A Uefcription of the Eye.

29. How Vifion Í5 cominonly explained.

30. Of the Paffage of the Light through the Hu~
mours of the Eye,

3 1 . JVhat we mean,, when we fay, that the Ima-
ges of the OhjeSls are imprejfed upon the Organs
of Sight.

32. How Vifion is performed.

33. Of Dioptricks.

34. Of Looking-Glajfes.

35. ASolution of fome Frohlems concerningVifton,

P A R. T II.

1 . F the Meaning of the Word Coffnography,
and the Ufefulnefs of the Science.

2. General Ohfervations.

3. Conjetures how to explain the apparent Mor
tions of the Stars,

4. Of the principal Points, Lines and Cir des,
whtch are imagined to he upon the Superficies of
the World.

5. Of the chief Ufes of the Cir des of the Sphere of
the World,

6. Ohfervations about the Sun’s Motion.

7. Conjetures how to explain the Phcenomena of
the Sun.

8 . Ohfervations am Conjetures ahout the fixed
^tars,

9, Ohferva-^

C

The C O N T E N T S.

9. Ohfervations ahout the Moon.

10. Conjetures whereb"j to explain the Pheenontena
of the Moon,

11. Of Eclipfes,

12. Of the true Bignefs of the Earth, Mom and
Sun, and oftheir Eifi anee f rom each other.

13. Of the Phcenoniena i^Merciiry and Venus.

14. Cosijetures for explainingthe Phanomena of
Mercury and Venus.

15. Of the Phesnomena of Mars, Júpiter, asid
Saturn.

i6- Conjetures wherehj to explain the Phesnomena
of Mars, Júpiter and Saturn.

An Explication of the Ph$nomena,upon Sup-

pofition that the Earth turns about its own Cen-

ter in Twenty-four Hours.

ly. A Caution about the Pales and theCircles.

13. An Explication of the Sun's Pheenoínena.

19. An Explication of the apparent Motion of fhe
fixed'Stars.

20. An Explication of the Motions of Mercury
and Venus.

2 1 . An Explication of the Motion of Mars, Jupi -
ter and Saturn.

22. An Explication of the Moon’s Motion.

23. Of the Syfiem 0/ Tycho-Brahe.

24. Refie tions upan the Hypothefes of Ptolemy,
Copernicus, and Tycho.

25. Of the N ature of the Stars.

26. Of Carnets.

2-j. Ofthe Influences of the Stars, and of judicial
Aflrology.

28. Of Gravity and Levity.

29. Of the Flux and Reflux of the Sea.

1,

P A E T

s

The C O N T E N T S.

P A R T iir

1. /'^F the Earth,

2. Of the Air.

3. Of ÍVater.

4. Of Salí.

Of Mineral Oil.

6 . Of Metals.

7. Of Minerals.

8. Of the Load-Stone.

Of fuhterraneoüs Pires and Earthquakes,

10. of Fountains.

11. OfWinds.

I 12. Of Mifts andCloüds.

13. Of Rain, Drizzle, Dew, and EveningDamps.

14. Of Snow, Hail^ and Hoar-Froft.

15. Of Honey-Dew, extraordinary Rain, and
Manna.

16. Of Thunder, Lightning, andFhunderlolts.

17. Of the Rain-how.

P A R T IV.

1 . F the Fhings contained in this Fourth Part.

2. general Defcription of the larger Parts
contained in a humane Body,

3 . Of the Brain, Nerves^ and Mufcles.

4. Of the Heart,

. Óf the Veins and Arteries.

. Of the Ladieal and Lymphatick Veins,

7. Of the Fongue and falival Jdudts,

8 . Of the Lungs.

9. Of the Liver,

10. Of the Spleen,

11. Of the Kidneys and Bladder.

12. Of the Motion the Blood.

í3* Of

The C O N T E N T S.

13. 0 / the Pulfe, or Beating of the Heart and
Arteries.

14. What 'Pinte the Blood circulates in.

15. O f natural He at.

1 6. Of Nourijhment and Growth,

17. Of the Animal Spirits^ and of the Motion of
the Mufcles.

18. Of Refpiration,

19. Of Waking and Sleeping,

20. Of the ConcoSiion of Meat.

21. Of the Motion of the Obfe,

22. How the Blood is made.

23. Of the Excrements.

O f Hunger and Phirft.

25. Of Sicknefs and Health,

26. Of a Fever,

ROHAVLT^

«

V

o F

Natural Philofophy.

P A R T. L

C H A P. L

'¡The Meaning of the Word Phyjicks^ and íhe Man-
ner of treating fuch a Suhjedt.

HIS Word, jftriaiy fpeaking, and i- The

accordingtocheEtymologyof ir, fignifies
no more than Natural-^ bur we here ufe Fhyjiáí,
ir ro fignify the Knowledge of natural
Things, that is, that Knowledge wbich
,j leads US to the Reafons and Caufesof eve-
ry EfFed which Nature prbduces, ,

2. But becauie we muíl firíl ftudy natural Philofophy, ?. Thatith
befóte we can be certSli whether there be anvfuchthing
as Phjfch or no • I fhould not proceed in a proper Me-
ihod, if I üiould here undertake to refolve this Difficulry.

■ B í Orall

3

RO H A U L T's System Part I.

I íháll not therefore at all infift on this, ñor any other
Queftions which are commonly callad previous ones. We
had beccer ac firfl: reniain in fome kind of Doubt about
thefe fort of Queílions ; but fuch a Doubt only, as oughc
not in the leaft to hinder us from uíing our utmoft En-
deavours to acquire this Knowledge, and to obtain thé End
propofed, without negleéling any Thing, that itiay ferve to
illuílrate the Truth, and explain the Effeóts of Natura.
y That the 3. One Thing we ought particularly to take notice of,

Mtimsoftie and that is, that all They who apply themfelves to theStu-
of Natural Vhilofophy,. are not Perfons utterly ignorant j
for by theirConvprfation withlearned Men, by readingof
Books'j by Experiments, and particular Obfervations, their
Minds are filiad with variety of Notions. But becaufe, per-
haps, theyhave given too much Credit to the Reports of
others, or perhaps have not throughly exatnined what they
have received by their own Seníés, or have impofed upon
themfelves by falíé Reafoning; therefore we are not to
thinkj that thcre is any great Advantage to be had frorn
that Knowledge which is got by thefe Means ; On the
contrary, it may be very injuriousj becaufe the Errors im-
bibed in our tender Age, before we could make a right
Ufe of our Reafon^ may caufe us to fall into ílill greater
ones.

4 . That they
éíight to be re-
examined.

4. Wherefore if we would proceed regularly^ we muft
lay afideallourold Prejudices^ and rejedt them as falfe j not
that we are immediately to embrace the contrary Opini-
ons as true, but only fo tp difpole our Mindsj as to give
Credit only to thofe Things which we have throughly
examined; and to begin natural Philofophy at the very Be-
ginning. But feeing this is a very difficult Task, and it
is hard to bring our felves to it, becaufe we eafily per-
fwade pur felves, that amongñ the Errors that have pri vate-
ly crept in, there have been alfo a great many Truths,
which oughc by no means tobe rejeñedj we will there-
fore go in the common Method, and recaining as ma-
ny, of our antient Opinions as we can, we will endeav-
our to lighten that Burden which cannot but be very heavy.
And we muft be very unreafonable indeed, if we will not
review our oíd Notions, and fubmit them to a freíh Ex-
amination.

C HAP;

Chap. 2. o/ Natural PHiLosopHy; ^

C H A P. II.

An Examination of the Notions that precede the
Studj of Natural PMlofophy.

T he Notions which precede the Study of Natural i.TheWMt
Philofophy.) tnay be reduced to two general Heads.

For firftj we know that there are Things exifihigm máyTe'Jm-
the World j and from henee we think weknow, at leaft
in part, -mbat they are. Thefe two Coníiderations are prin-
cipally to be attended to, that our propofed Examination
may be as univerfal as poffible. Lee us firft fee what Mo-
tives there are to induce ustobelievcj that there arecertain
Things xsú\y exijii?!g in the World j and then letus fee what
í Reaíon we have to believe them to be fuch as we judge
thetn to be.

2. And to begin withourown felves,- we know by ex-
perience, that we are capable of diverfe Thoughts, which
cannot be in us, but they muñ be perceived. The Ictea ontown ex-_
of Exijíenceis oneof thefeThoughís i and ouruaturalRea- ,
fon teaches, that Nothing can have no Properties, and that
■what thmks, mufl exifi. Henee it is plain how we come
by the Knowledge of onr o-wn Exiflence. For every Man
muñneceífarily reafoninthismanner: 1 think ^ that which
thinksmuftof neceflity j therefore

3. AManwho comes to the Knowledge of his Exift- That onr
ence in thismanner, knowshimfelf only to be fomething
that exiñsj the Itiea of which does not inelude Extenfwn fioner than
in it. It is truBj he may have an Idea of a Thing extended
into Lengthi Breadth, and Heightj but becaufe this Idea tmrJiiy7C-
does not at all inelude Tbought in ifj the Thing that thinks, Jii»a T/iingii
and the Thing that is extended, are to be looked upon as
two Things really different from each other ; and there is
no Reafon hitherto for fuch a Perfon to think himfelf an
extended Thing. And hncmk That which thinks, which
is in usj which we know before all other Things^ which
we imagine not to be extended, is what we cali our Sotd
or Spirit, and That which we conceive to be extended in
Length, Breadth, and Height, and to which we imagine
Thought does not belong, is what we cali our Body j it is
evidenr, that our Soul cff Spirit is known to us fooner than
our Eodj,

' - J B 2 A?

4^ That
have no other
knoTvUdfieof
ihe Exijicnce
of thofe 3o-
dies of vohkh
the World is
com^ofed, bnt
by the dijfe~
rent soays of
knowing Chat
are in 7<Ji
y, What thefe
U'ays of
Knovoledge
are.

6. fVhat is
meantby Per-
ceptíon or l-
magination.

fi Whatts
meant by
Jíidgement,

Z.What is
meant by Rea-
fin,

What is
meant bySeri'
fation.

1 o. JhatPer-
ception alone
is nota fnjji-
cient .Ajfu-
ranee of the
Exijience of
any Thln^,
í f . híeither
dees 'ftidge-
went alone
fuU} i cnvitice
MSaf the Ex-
ifiroLt oj
Tningí,

RO H AU í System Partí,

4- As to thofe Bodies of which the World is compofed,
(amongft which our own is to be reckoned) k is certaia
we cannot know that they exift, but by the diffirent Wajs
of Kfwwledge which are irr us j and in order to know if we
haye madea righ: Ufeof them, we will here confider each
of them diftindtly.

5- The difFerent 7 %/íof- Knowledge that are inusj may
all be reduced to thefe Four ; viz. Per ception, Jndgement,
Reafon, and Senfatim.

6. By Perception is meant íimple Apprehenfion, or the
fimpie Idea which we have of Tbings, withouc afSrming
or denying any thing concerning them; whether this Idea
raifesany Image in our Minds, and foiscalled hnagination,
or raifes no Image, and fo has only the general Ñame of
Perception gwea to it... Thuswhen we hear the Word Tíw,
the Idea which we then form in our Minds, is an Imagina-
tion; but when we fpeak of a Thing which cannot be
reprefented by any Image, as of Houbtfulnefs ; the Idea
which we then have, is only limpie Perception.

y. yW^eMesíisthejoiningordisjoiningof twoThingsby
theMind, when, accorqing to the diíFerent mannerof its
conceiving them, it affirms or denies theone or the other.
Thus whenwefay, that the Earth is roimd, wejoin toge-
ther the rwo Things which we underfland by the Words
Earchand Roundnefs, and this is co&eáyudgemettt : So alfo
when we fay that the Earth is not round, that is, disjoin
thofe Words; thisis alfo called

8 . Reafon isa Judgement that depends upon a former
Judgemerit. Fur Example: AfterI have judged, that m
even Number can be compoutided of five oddNumbers, anti al-
fo, that. the NumberT'ujefity is aneven Etumber, and thence
conclude, that the Number Twenty cannot be .divided hito
five odd Numbers-, this is called, Peafoning.

g. . Senfation, is Touching, Smelling, laf ing, Hearing and
Seeing. ■ .

10. Firft, it is evident, úiztÚstharePerceptionoí aThing
is not ÍÜfficientto con vince us that the Thing it felf exiíls;
for Inñance, becaufe I co-xt conceiveiíTriangle, it does by
no means from thence follow, that a Triang/e exifis.

■.II. Itis certainalíb, that our alone is not fuf-

ficient to convince us of theExiíIence of any Thing. For
thoügh we cannot help paíling our Judgement upon many
Things ; for Inílance, That iftwo Things be equ.il to a Third, '
they' are equal to each other’, that if i^-'uals be added to Eqna/s, .
their Sumsviill be equal, &c, notwithftanding wliich, we
-do not certainly know, that any Things that are equal or
■ ■ ■ , ame-

Chap. 2. ef N ATURAL .P HILOSOPH Y.

unequal exiftj and the Truthof o\ix Judgement zgxQss oúy
to the Things that may poffibly éxift.

12. We may alio innnirely yarious ways,- and by
this means all the Mathematical Truths are diícovered,
which are lo. difFerent from one another, and from the
Principies from which they are deduced : But becaufe the
Confequences have a ftriíl: Relation to the Antecedents, and
can contain no morein them than they^ and wehave ab
ready feen that our Judgement does not prove that any Thmg
exifts^ it followsj that our i2eij/ow/V¿ proves no more than
this, that Things without us may poffibly exiíl.

13. However, ' thereisssí'Exceptionto this Rule, and
that isj God: For whoever has the Idea of Him, may by
Reafon be aílured of his Exiftence, if he be confidéred as
a Being every vvay perfedb, and if Exiftence be owned to
bea Perfediion. Butlffiallnot hereenterinto the Párticulars
of this Demonftration,- theDignityof this Subjedraerits
to be treated of particularly by it felf

14. But lince weare herefpeakingonly of natural ThingSj
and our Feneption, Judge77ient, and FeafoTi alone do not
prove their Exiftence, we muft certainly have recourfe to
our Se7ifes before we can judge that they exift. And we
cannot know whether our Senfis do fufficiently prove this,
ñor in what manner they prove it, unlefs we firft define
what we mean by SeTifaüon.

15. Long Cuftom makes us many times reaíbn with fo
much eafe and readinels, that very ofcen, ReafoTt and Se»-

go together, when we think that Se 77 fatio 7 i only is
concerned.- Wherefore that we may not confound the one
with the other, and fo be led into Error, let us examine
thisMatterin other Perfons. Let us fuppofe a Man juft
born, and that he v/as in an extraordinary manner endued
with the Judgement and Prudenceof a grown Perlón 3 and,
that we may examine only oneSenfeat a time, let us fup-
pofe that his Eyes are not yet open, and, that he is pqt
into a Place, where there is no kind of Smell or Noife,

16. Now inorder to find out v/bat the SeTifi of FeeHng
is,- let this Man’s Arm be prick’d with a Needle. It is
manifeft, that he will feel the fame fort of Pain that we
feel, when at any time we are pricked with a Needle,
becaufe we fuppofe him tp be fucb á fort of a as we

II. That
Reafon'does
not convizicc
lis that any
Thing exifís

without OUT

jelvci.

1 3 , The Ex^
ijteneeoj God
may be pro-
ved by Rea-
fon.

I4. That toe
onght tu malte
ufe of onrSen^
fes to pTOVCy
that the
Things with»
out Ms exif.

ly. The Way
to hnovo di-
jiinfíly what
Senfation is.

jS. Í 4 n Ex-
ampie in a
Needle- ■.

I There is one Uxeeption^ How the
Idea of God proves his Exiftence,
See Cartef Princip, Part, i fíkrtic.
14. and Regis Mctaphyf Lib. I,
Parí* I. Chaf.f, But thisis coonice

and fubcle an Argumenc j chatdrawn
from the Vanecyj Beáut]?5 Orderj ánd
Difpofition of the Création,^ do^s
much more fully and ftronglyinfer-f»
Godi

6 RO H JUL'^’s Systiíu Partí. •

are ; Now abílraótedly from any Judgment or Reafoning,
je is evidenCj that Sevfation in this Man is nothing elfc)
buc the being afifecled with a certain Pain, which belongs
to himfelf only. So that if any Perfon were ib weak as
to believe, that a like Pain was in the Needle, we íhould
certainly know for all thatj that it was not the very Pain
which the Man by Se?ifatio7í felt.

17. That ví j y. Let; US make fome Refledion here : In the Senfation
'í"gfmdm- now Hientioned, there are four Things obfervable ; Firft,
eA(«¿ e¡fe. A Man capable of Senfation : Secondly, A Needle, or the
Objed that railes the Senfafon : Thirdly, The ABion of a
Needle upon the Body, in which it produces fotne Change :
Laftly, The EffiB of the ABion of the Needle, atid of the
Eajjio'n of the Bodj, natnely, the Vncking, or the Pain.
Now fince’tis this Laft only that is known, webuíl con-
clude, that this Senfation not being attended with any
Judgement or Reafoiij is nothing elfe but a tonfnfid Per-
cejition ariCmg fiom íhe new State of theMind, which does '
not any way make known to us this new State, ñor the
external Objed which caufesit, andis the Occafion of the
Senfation.

iS.Wi's 18. From what has beenfaid of that which iscau-
ampie teaches fed by a Needle, it is eafy to apprehend the fame thing of
*^^0 other Sort of Senfations, fuch as Feeling, Tafling, and
' Feeiing,Taf.- Smelling. For fuppofc the nalced Arm of the forementi-
oned Perlón to be lightly touched with a Feather, or
■’á 2ny other foft Thing ,■ fuppofe a red-hot Goal, or a Piece of

Ice to be laid on any part of his Body • fuppofe a Drop of
Wine poured onhis Tongue, or a Rafe, orany other fweeu
fmelling Thmg put to him ,■ we can eafily underftand, that
the Tickling, the Heat, the Coid, the Tafle, and the Smell,
which this Man perceives, are all wlthin himfelf, and be-
long to him in the fame manner as the Pain did.
ig. Aridotle ^ 9 - And fince there, is noReafon why we fhould think
hadgoodRea- dilFerently of the Senfations of Hearing and Seemg than of
others, we may loóle upon it as certain, that Sound, and
tnmdPaf- Idght, and Colours, arcas much in us as Pain or Tickling.-
/o» wíi-e the Wherefore we may fay with Arifotle, i that all Senfation ii
' a khídof PaJJion, and when we have any Senfation, what,-

ever fort it be, we know very well what the Objeds raife
ín US, but we don’t know what they are in therofelves.

í. Añftoc. de Anima. Lib. i.cap.
j. Senfation conjifis in keíng put iíito
Mation^ and is a fore oj PaJJion^ as
was faid^eftrej fir' there fsems ta be

fome Change or Alteration made in
as, again, chap, 11. Senfation. is
a foTl of Paffion.

ao. Bul

Chap. 2. of Natural Philosophy. -j

20. But chis is not the general Opinión of Mankind, lo. ^ wU
who, on the coritraryj are apt to think, that the Sound

which they heatj is. in the Air-, or in the founiing Body as
they cali it ; fo alfo that the Light and Colours which they
fee, are in the Blame or the T^peflry which they look upon ;
and the Reafonof itisthis, becaufewe donotfeel ' Sound,
and Light, and Colours within our felves, as we do Fain
and Ttckling, but afcribe thein to external Things,- and be-
lides, the Colours which we fee, oftentitnes leem to be
much bigger than our felves.

21. But to Ihow that thefe Reafons are not of any ^s.Thscm-
Weightj we need only confider, that very often we have

a Perception of a Multitude of Things, which we think mmyExft^,
are without us, and are a great deal bigger than our felves, riments.
when at the fame time there really is nothing without us,
that ísthe Caufe of that Perception.

22. Firfl, In Dreams we very often hear Sounds, and'^.*-
® fee CoIourSj in the fame manner as if we were awake, and

we alcribe thoíé Sounds and Colours to external Objeñsi
and we imagine thofe Colours to be much larger than our
felves; though there isindeed nothing without us, to which
they cantrulybeafcribed.

23. Secondly, Perfons in a Phrenlie, or in a vident 23.11. R*-
Fever, fee alfo Things without them, which really are

not fo.

24. Thirdly, We often hear a Finging in our Ears, or a 24 . iii. Ex-
certain Sound which we judge to be at a great diftance,

when the Caufe of it is very near us.

25. Fourthly, A Caridle, orany other fmall Objedt, at a zs-y^-Ex-
little diftance, appears double toa Perfonin Drink; or ¿f í*’'™"''*
we prefs the Córner of our Eye with our Finger ; fo that

there will then appear to be two Objedts, when we certain-
lyknow, that there really is but one.

1 . Soimdy and Lightt and Colours j
&c,) In order to accounc for cheíe
Prejudkesj we may óbferveí i.Thac
pjjin and Tickjing do much more
firongly affeíb us, and make a greater
Changein the State of the Mind, than
Sound, and Light, and Colours j lo
that they ^re fooner and more eafily
taken noticeof, and ímagined tobe-
longtous, andcobeinus. 2. When
Sound, and Light, and Colours, are
at firft perceived, there is always
fomethlng before us, that upon.
US, and to which we afcrx* tbem, t
$uc PaíA and Tickiing ofceo ariíe

from an ínvtfible Alteración of tha
fmall Particles of che Body, that is,
from a Caufe at firft unknown tous':
Therefore we are a long while ufed to
look upon chefeas fomething in usi
*cill there appears to be íbmeching
without US, to which they may be
afcribed; and afcerwards, when we
do fometimes experience, that they
proceed from various externalThings,
we are ñill apt co think, that they
are not i n thofe external Things, bu:
in our felves, becaufe we have been
ufed to t^hink íb.

B 4

26. Fífihiy,

8

ROUAUL't’s System

Part 1.

ȇ. V. %x-
fcYímení,

26. Fiphly, If in the Dark we wink with our Eyes up-
en the Fíame of a Candle at a little diftance, we íhall ima-
ginCj that we fee Raya of Light, which feem to ílream
from the Fíame upwards and downwards in the Air ; and
yet there is no doubc, but that thofe Raya arife from the
Senfation of him that perceivea them, and that out of
him they are nothing,- it we confider^ that other Perfons
who look upon the Candle at the fame time, do not fee
them ; and the Perlón himfelf who fees them when he
winka, ceafea to fee them the Moment-that heopens his
Eyea, and looka more intently.

t-j.ThereU - 7 ; We Ihall be morefullyfatisfied, that thefe Raya are
fimtthingre- Hot in the Placc that we imagine them to be, by this
’thUEaeH- ^°nfi‘^^''2tion ; If they Were there, it would follow, that up-
Tttcnt!’'"' onputting a dark Bodybetween the Eye and the Place where
they appear to be, theymuñimmediately vaniíh j but they
_ do not vanilh, but onthe contrary arefeen ftill, only a little
nearer, viz. between the Eye and the dark Body that interpo-
fea. But that which ia moft obíérvablein thia Experiment,
is, that if the dark Éodyberaifedby little and little, as if the
lówer Rays wefe intended wholly to be hidden by its In-
terpolition, they will be ftill feen, when the upper ones
wholly difappear j which could not be, if the Rays were
really in the Place which they feemto be in.
aS. VI. 2%. Sixthly, We fee the Colours through a triangular
periment. Glafs Prifm, very bright, and exaétly like the Colours

in the Rainbow,- thefe we certainly know are not where
they appear to be.

29 . Oi this kind are the Experiments of Looking-Glaf-
Jes a7td Mttltiply'mg-GlaJJh, which reprefent Objecfs to us,
where we are fure they are not.

30 . We muñ not here omit an Experiment of thofe
Períbns who have loñ ’any of their Limbs, an Arm, or
a Leg, who, many Months, and íbmetimes many Years
after they are cured, feel frequent Prickings, and other
Senfations, which they cannot help judging to be with-
out them, vtz. in thofe Places where their Fingers or
Toes would have been,- if they had not been cut ofF.
This Judgementisevidentlya Miftake, it being certain, that
this Senfation is within themfelves, and not where they
take it to be.

31. This Experiment, together with all the foregoing
ones, plainly ftiow, that we have within our felves the
Senlations of many Things, which we cannot help think-
ingare without us, though they reC?y are not; and were
it not for the common Way of Speaking, which is theu-

fuai

29 .yn. Ex-

periment,

5o;Vni.EA-

periment*

y. A Dlf-
peulty vohích
arifes from
ihc eontmon
Cujiom of
.^eakiii¿i

9

Chap. 2. of Natural PhilOsophv.

ílial Reafon given, we ought wlíolly to lay aíide that vul-
gar Nocion, which we have entertaiued in our Minds
from our Infancy, w*. they are withoin us. For
(tnay any one fay) as he who touches a Stick, has reafon
to believe, that the Stick is fomething without hi'm that
touches ít^; fo when any one fays, that he fees a Colour,
he has Reafon to fay;, that the Colour which he fees, is
fomething different from him that fees it, and belongs to
the Objed;.

32. Butitis eafyto getclearof this DifEculty, if weob- %í.Thtum.
ferve, that all Languages do not afford equal Plenty of
Words upon every Subjeft. Thus for Example, in the f¡amed.
'Latm Tongue, the Word Animal is üfed to exprefs
the Kind, under which the whole Species of Animáis is
contained^ the Words Man and Horfi, are ufed to figni-
fy thofe Species," and the Words Teter and Faul, Buce~
fhalus and Bayard, to fignify the Individuáis of thofe Spe-
cies: But the Cafe is- different in the prefent Subjedtj we
* ufe indeed in our Language the Word Senfation, by which
we underlland, in geiierah every Perception which we have
by the means of Bodies ,• we have alfo the Words Beeling,

Tafimg, Smellingi and Hearing, tofignify the particular Spe-
cies of thofe Senfations," but if we would defeend to any
thing ftill more particular; we then want Words, and are
forced to make ufe of a general Ñame, with which we
only joyn fume other Word, to determine its Signification :

Whence it follows, that when we fay, forExample, that
•we feel the Heat, or that •we fee the Colour if we forbear
Reafoning about them, and attend only to the bare Sen-
fation the Veeling ought no otherwife to be diñinguiflied
from the Heat, ñor the Seeing from the Colour, than in
any Species, ' the Gemís is diftinguifhed from the
ronce: For the Colour and the Heat are Senfations which
belong to our own felves only, and are nothing more than
our own Perceptions.

I.' The Gennsis (ii/iingitijheíij &c.)

The Author’s Meaning is this^ that
many People are led into Error, by
the Porms of fpeaking ; as when by
reaíon ot cheFewneísofWordssOur
Meaning cannoc be exprefled buc by
itaore Words than one j chiis when we
íay, úxztvoe fee Reii?tefíi orfeeLHeaf,
ihey fo underñand k? as if by one
of chefe Words we intended to figni-
fy the Senfation it felf, and by the
other, to fignify fomething ^^^out
US, which is che Caufe of that Sen-
fation. Nüw if whac wecall/eW»^

' ' ' 33. Thougb

Rednefs^TLuáfeeling Heat couldbeex-
preíTed by one Word, asP^a/», which
is che fame Thing zifecling Páin, or
Tickling-i which is the íame as feeL
ing TukUngi are expreííéd by one
Word ; v/e fhould eafily apprehends
that che Rednefs which we perceive
by our Sight, and the Heat which
we perceive by our Fecling, are no
more withoutus, than che Pain which
we feel when our Arm is pricked
witha Needles or che when

ic is couched lightl^ with a Feather.

30 ROH JULT’s S Y STiM Partí.

fi^ihytÍTré ^5- Though I haí^c been already toolong in lliowing
w betwixi vihat we perceive Jimply by Sight, is luholly wtbm our

^ghtMd felves i I would yec malee appear the entire Conformity

“ there is becwixc Seeing and ’Eeeling. Let ns confider thenj

that when an Objeót of ’EeelingíSe.díS the Body but lightly^
it raifes in us indeed a real Senlation, but it is fo yveak
an one, that it is gone as foon as the Objedt ceafes
to touch the Organ of Senfation j fo likewife, if the Ob-
jecl of Sight be weafc it is no fooner removed from our
£yes, but we ceafe to feeit. And as an Objed of Veel-
ing, which flrikes us with a greater Forcé, excites a Sen-
fation, which remains afterit isfeparated from the Organ j
in the fame manner alfo, a very ftrong Objed of Sight^
raiíes a bright Senfation, which continúes for fome time,
though we do not look upon it, but turn our Head ano-
ther way. Thus if any one looks full upon the Sun, and
immediately goes into a dark Place, he will fee the Sun
there, and fome Sparklings of it.

hl'vJmade^ 34- From what has been faid coHceming our and
«pTorpverai Manner of Senfatmiy it is evident, that they make
Meánsef knowH to US only what is in us, and belongs to usj it is
Ki‘onUdge jjg certaiii, that they are not alone fuíBcient to prove
conviscedthat to US, that any 1 hing at all exiíts without us which does
Thmgscxifi not bcloiig to US," and this having been already íhown of
tn OH as. gr^g^y particular Means of Knovoledge, we muft neceífarily,
conclude, that we have made ufe of feveral of thofe Means
in order to be convinced that Things do exift without

US.

Sj.m Ue- 35- The Method wefeem to have proceeded in, isthis.
thodnhkhne Firftj Senfation; Next, weobferve, That this Senfation is
frooie m. fometimesinour own Power, andfometimes not : Whence

we infer, that we our felves are not the fole Caufe of our

own Senfations j that we contribute fomething towards
them, but notfomuch, but that wedepend alio upon fome
other Caufe,- andfowebegin to fee, that we do not exift
alone, but that i there are many other Beingsexiftingtoge-
ther v/ith us in the World.

qS. Thenx- Whocver acknowledges this Truth, muft confels,
¡fimoe of that he has been in an Error lo long as he thought that the
^ahkhyfm' Exiñence of Things without him was proved by his Sen-

Senfes, is

rnade htoron I . There are many other ’Beings, &C.)
io tff princi- Buceventhjs does no^, leem fuffici-
pallyhyRea- ently to dei-nonñrr.teí thac corpórea!
jonin^. Things exift: and indeed it docs
not feera capabie of a ítrict Deraon - .
firation, See ^^albranch. ^nnou
10. ‘Rock. í. of hh Smehafter

fes 3

Truth* We muft acquieíce in this >
That God has not created usin fuch
a mannerjthat every Judgement which
weV*Jtike of Things exiftiíng wóthout
us> íhould be inevitably falfe. Se©
Cartef Fmcip*^(irí 2» i.

II

Chap. 2. of Natural Philosophy.

fes ; for all that thefe can do, is only to be the Occafion of
knowing them j and ic is chiefly from Reaibning that we
areaíTured of their Exiftence.

37. In the fame manner aswe conclude from one fin- 37. awwe
gle Senfation, thatoneThingexifts; we conclude alfo from
different Sores of Senl'ations, that there are differenc Things
exifting j all which, becaufe we imagine them to be ex-

tended in Length, Breadth, and Thicknefs, we cali
Bodies.

38. Amongfl: thefe Bodies, there is one which we con- ?s. Hm me
fider differently from the reft, and are obliged, in a fpeci-

al manner, to look; uponas our ownj not only becaufe it OHT omi 'Body
is always prefent with us, but alfo, becaufe, when any >« {‘irtUuiar.
Alteration is made in it by other Things, it caufes certain
Senfations in us,- and on the other hand, certainThoughts
in us, produce certain Alterations in that. Thus if 1 will
to move my Arm, it is prefently moved j but if I vilU
® to move another Body, that will not be put into motion
by my Will alone.

39. Wemay further obferve, that after the foregoing í^-We ^re
Refledions have convinced us that our Body is compo-

fed of many different Parts, fome of which are the Or- as many
gans of Senfation j the different Senfations we have, are
no longer a certain Proof of the Exiftence of a Number "íf"asme
of Things without us : For there is juíl Reafon to fufped, haaediffnmt
that the fame Objed may raife different Senfations in us,
byadingupon different Organs-, and therefore though the
Fire by affeding our Eyes when it is at a great diftance,
raifes the Senfation of Light j and when it is near, raifes
the Senfation of Heat by affeding our Hands; yet we
cannot from henee colled the Exiftence of more than
one Objed.

40. There is another Miílake contrarytothis, which itis 40. ..íPre¿
eafy to fallinto, and therefore ought to be avoided. For,

doesit not feem reafonable to determine with Aífurance certain af a
the Exiftence of many Things, withoutanydangerof be- 0/
ing deceived, if in making ufe of but one Senfe, and em-
ploying it in but one manner only, it reprefents to us ma-
ny Objeds at the fame time Now that we may not be
deceived here alfo, we ought to confider the Médium
through which the Adion of the Objed is tranfmitted,-
for Example, a muhiplying Glafs makes us fee many Ob-
jeds at once, when there is only one that really affeds
our Eyes j which that here alfo we may be de-

ceiyed.

41, Thefe

12

41 * TTfs Sf¿m
nification of
the Kames
sohich roegive
to many
Things,

42. w4 Mi[-
tabeabout the
Slgnification
^ mrds-

43 » lAnother
hiidabe*

RO H AU L'T’ s System Partí.

4.1. Thefe two Obfervations teach us, that we ought
not to judge rafhiy, ñor at firíl Sighr, that a Number of
Things exift ; However:, afcer having talcen all the Precau-
tions requiíite, when we are once plainly and fully cori-
vinced of their Exiílence, by raeans of the different Sen-
fations which they raife in us; we cannot help arguing frdtn
the Añ to the Powerj as Philofophers cali it, which is-
very natural to all Perlbnsj and thence concluding, that
thofe Things have within thetn a Power to afifed our
Senfes : And henee it is, that we give Ñames to thofe
Things, fignifying fuch different Powers. Thus a Body
which raifes Heat in us, we cali á hot Body j and the bare
Power of raiíing this Senfatioh in us, we cali the Heat of
this Body.

42. Whence it is plain, that they are deceived, who, be-
fóte they haveftudied Philofophy, underíland thefe Words
in a larger Senfe than was faid befóte ; for example,
who, wheh we mention the Heat of the Pire, imagine f)
p'refently that there is fomething, I know not what,

in the Fire, like that Heat which the Fire raifes in us j
for the giving of a mere Ñame only to a Thing unknown,
does not at all malee that Thing known to us.

43 . They alfo deceive themfelves as fooliflily, though
to appearance, they are more acute, who, in order to
prove that there is in the Fire fomething, I know not what,
like that Heat which it excites in us, bid us go near it
and try : Noyt, though we go near it a thoafand times,
nay, though we were fcorched by ir, all that this de-
monftrates, is only what the Fire does to us, and not
what it is in it felf. When we fpeak therefore of the
Heat, or Co/d] or Smells, or Soiinds, or Ught, or Colours of
Bodies, to fay, that they are reallyr¿/zz¿f which are properly
Objeóts of our Senfes, is a great Miñake. For he who
fays' this, mufl: imagine, that we come to the Know-
ledge of them by bare Senfation only, wiiich is abfo-
lutely falfe.

Chap. 3* o/ Natural Philosophy. ig

CHAP. III.

\

’^he Manner of appljing Philofo;phj to particular
SuhjeSls.

T H E Obfervation which we have now inade, is of i. m muji
fo grea: Importance, that it alone íliows us the true
Method of Philofophy on particular Subjeíls : For frotn
henee we learn^ that in order to find out what the Natura
of any Thing is^ we are to fearch for fotne one Particular
in it, that will account for all the EfFeóbs which Experi-
ence iliows us it is capable of producing. Thusí if we
would know what the Heat of the Fire is, we muft en-
deavour to find out fome particular Thing, by means of
which, it is capable of producing in us that Sortof Tick-
• ling, or pleafant agreeable Heat which we feel at a little
diftance from ic j and thatSort ofPain, orfcorchingHeat,
which we feel when we approach too near it ,• and the
lame Thing mufl: alfo explgin to us, how the Fire comes
to rarify fome Bodies, and to barden others, and to diífolve
odiers: In aword, itmuíl: explain allt'he Effedis that Fire
produces. And in order to this, we are principaily to
guard againfl: any Prejudices we may have entertained con-
cerning it; and not immediately to imagine, that thereis
’ in the Fire the fame kind of Heat, whether pleaíánt or
fcorching, which we feel, when at a diftance, or near to it.

For indeed, there is no more reafon to attribute fuch fort
of Heat to the Fire, than there is to aferibe the fame
fort of Pain to the Needle, which we feel when we are
prick’d by it ^ and as he would without all doubt be de-
ceived, who fhould aferibe the fame Pain that we feel to
the Needle j and would after this, labour to no purpofe,
in trying to find out the Nature of it ; fo likewiíe would
it be in vain, after having aferibed to the Fire that fort
of Heat which we our lélves feel upon that Occafion, to
attempt philofophically to explain the Nature of Fire;
for nothing folid can be built upon fo bad a Foundation,
nothing but Conjetures and Chimera’s.

3. What is now faid of Heat, may be applied to all 2. r-rkm
other Things : And by this Rule, every Thing hereafter is
to be examined. If that which we fix upon, to explain mí '

the particular Nature c^any Thing, do not account clear- «»<•.

ly and plainly for every Property of that Thing, or if it
he evidently contradited by any one Experiment ; then

we

S4 ROBAULr>s SlrstÉM Partí,-

we are to look úpon our Conjedure as falfe; butifitper-
fedly agrees wíth all the Properties of the Thing, then
we may efteem it wpll groundedj and ic may pafsfor ve-
ly probable.

3. w'í muji 3. Thus we muft confent our felves for the moñ part,

Things may be j wichout pretending co come
ttnt nhh to a certain Knowledge and Determination of what they
írohabiiitj. really are i for there may poflSbly be difíerent Caufes ca-
pable of próducing the fame Effedj 'which we have no -
Meaos of explaining.

H.WTiena 4. Now as he that undertakes to decypher a Letterj
m’ayheaihm- Alphabet fo much thc morc probable, as it

edverypo- anfwcrs to thC Words with the feweft Suppofitions j fo
iahic. may afSrm of that Conjedure concerning the Natura

of any Thing, that ic is the more probable, by how much
the more fimple ic is, by how much the fewer Properties
were had in view, and by how much the more Properties,
different from each other, can be explained by it. Thus,
for Example j if having taken notice only of four Pro-
perties of a Thing, we form fuch a Notion of ic, that the
Conjedure we make to explain them, will hold as ftrong
for twenty Properties which we find co be in it j ic is cer-
tain, that thefe areTo many Proofs that our Conjedure
is very good.

y. Whma j. And indecd there may be fo many, and fo very dif-
Cmijedrm « ferent Properties in the fame Thing, that we Ihall find it
i'eaihw^áfor Very diíHculc to believe, that they can be explained two
tíTrath. different ways. In which Cafe, our Conjedure is not
only to be looked upon as highly probable, but we have
Realbn to believe it to be the very Truth.

6. Wcmght 6 . Lañly, To pre vene any Scruples that may afcerwards
»nt too íafdy arife, we muíl confider, that, if our Conjedure be other-
aCmjcVuTí wife wcll grounded, it does not lofe its Probability, be-
thatis wcii caufe we cannoc upon the Spot explain by ic a Property,
irmnacd. which appears from fome new Experiment, or which we
did not befóte think of : For it is one Thing to know cer-
tainly, that a Conjedure is contrary to Experience; and
another Thing, not to fee how it agrees toit; forthough
we do not at all lee the Agreement, ic does not from thence
follow, that ic is repugnant. And it may be, though we
don’t fee ic to Day, we may fee it to Morrow ; or others
who can fee further than we, may at one timeor other diC-

• T^ri II. cover it. Thus, as we ihall fee * afterwards, Telefeopes
vdiich were not in ufe till our have confirmed the

”«•7. Nypothefisof Copernicus, micerning t he Motion of Venus atid
Mercury, which feemed not very well to agree with the dif¡-
ferent Magnitude of Venus at different times. Chap,

Chap. 4 - o/. Natural Philosophy; 15;

CHAP. IV,

A Caution concerning Words.

S I N CE we are accuftomed to conneft our Thougíits i . That m
with our Wordsj and ofcentimes attend more to the
Words than to the Things fignified by íhem ; tbat we utanlng w«
may not for the futurebe ledinto Miílakeby Words, we
fhall not make ufe of any here, ñor have regard to any, ‘
whofe Meaning we do not clearly underftand. Where-
fore in this Treatife we íhall wholly negledt fuch fpecious
Words as Antiperiflafis, Sympathy, Antipathy-) a Dejire of
Union, Contrariety, and the like. And as we do not ule
them our felves, fo we íhall have no regard to them frotn
^ others, unlefs theytellus, very clearly and diftiníHy, whaC
they mean by them, and how we are to underftand them.

Leñ therefore we Ihould fall into that Fault which we
condemn in others, we íhall here define the Terms of Art,
which, after the Example of moíl Philofophers, we íhall
make ufe of.

2. The Word fignifiesonly that which wor wryír; í.mati¡
for that which does not exift, is indeed nothing. For if
any Thing be to exift next Year, we may affirm, that at
prefent it is nothing, and it is only the Idea which we
have of it, that is any Thing.

3. We underftand by Suhflance here a Thing which we 3. Whatis
conceive to fubfift of it felf, independent of any other
created Thing: Thus aPiece of Wax is a Subftance, be- “-f
caufe we conceive it to fubfift of itfelf, independent of
any other created Thing.

4. Obferve here, that I don’t fay abfolutely, that a Sub- 4- Tíáí we
ftance is a Thing which fubfiñs of it felf; but that it is íf-

a Thing which we coftceive to fubfift of it felf, which I fay Lrdíngtocur
on purpofe to make this Definition of ufe. For though Usas of them.
I know very well, that our Conceptions or Imaginations
lay no Neceffity upon the Things thesnjelves) yet they are
neceílary towards ourjudging of them, becaufe we know
Things from our Idea’s only, and we oughtalways tojudge
according to our Thoughts.

5. We cali that n Mode, or Matmer cf Exijiing, or an Whatis
Accident, which we co^^ive necefParily to depend upon “

lome Subftance. Thus, becaufe we cannot poffibly con- A '
ceive the Roundnefs of a Giobule of Wax to fubfift with-

x6

ROBJUL r^s System

Partr,

4 * Tbat a
Mode cannot
be tramfcrred
from one ^nb-
je¿í to ano-
sher.

7. What is
meant by a
^aliíy»

8. That the
vpord ^ality
has not a de-
termínate
Slgaificationt
but is hovoe-
•ver stfeftil.

9. What is
meant by the
VVords Ver-
irte cr Facnl-
ív.

ro. What
the EJJcnce of
A Thing is.

IJ. VVkat
the ejjential
Praperty of a
Thing is.

out the Wax, therefore we cali k a Moí¿e or Bamer, of
exifing, or an Accident.

6. From whence ic follows, that a Moé, or an Acá-
dent, cannot be transferred from that Subftance which is
the Subje¿t of ic, to any other Subftance; for if it could,
ic would not. chen have depended entirely upon the firft
Subftance when ic was in ic, which is abfurd.

7. By the Word f^ality we mean that, by which a
Thing is denominated-yicÁ ; Thus that in the Fire, what-
ever ic be, w'hich has a Power to raife the Seníation of
Heat in us, we cali a fduality of the Fire, becaule ic is
from thís that the Fire is laid to be hot. -

8. That which is to be feared here, and which hath
made lome over-fcrupulous Perlbns wiíh that chis Word
were never ufed, but wholly fupprefled, is, that lome Men
fooliftily think, that they are very knowing, if they can
but apply chis Word, and fome other of the like Sort, to
exprefs a Thing which they do not at all underftand.
However, I cannot agree to them, but think it fufficient,
if we do not ufe ic in a bad Senfe. For it íéems to me
(as ic did formerly to Ariflotk) to be very properly ufed
for that in general, whatever it be, which we conceive
to belong to a Subjed, and on the accounc of which, we
give a particular Ñame to ic. Thus, untilwe clearly and
diftindiy underftand what che Hcat of the Fire is, we may
cali it a -^ality of the Fire.

9. The Words Vertue ox Facultyy in any Subjed, figni-
fy in general, the Power which a Thing has to produce
fome'EíFed in another Thing. Thus what we juft now
called a ^ality, upon this Account, that the Fire is from
thence denominated hot ; may alfo be called a Vertue of
the Fire, if we coníider, that it is from thisy though we
know not what it is, that the Fire can heat any Thing.

10. The EJfencc of a Thing, is that which ic principal-
!y is, or that which confticutes the FÍaturé of it, and by
which it is what ic is ; Thus the Ejfence of a right-lined
Triangle confifts in this, that ir is a Figure termiriated by
three righc Lines. From whence it is evident, that al-
lowing che EJ[e7íce of a Thing, is allowing the Thing it
felf; and on the contrary, talcing away the Effence, is ca-
king away the Thing ic felf.

11. We cali that an eJfentialEroperty of a Thivg, which

we conceive fo to belong to the Thing, that it is che

neceílary Confequence of ics EíTei?’! ; Thus, that any two •
Stdes togecher, are longer than the Third ; and that the
three Angles are equal to two righc ongs, are Properties

that ■-

Chap. 4- Natural P HiLOSoPHy,'

thac belong tothe EíTenceof a Triangle; becaufe thefe fb
belong to iCj thac they are a necelláry Confequence of a
Figure’s being terminaced by three right Lines. So iike-
wil'e it is che ejfential Fro^erly QÍ a righc-angled Triangle,
tü havethe Squareof theSide oppofed to the right Angle,
equal to the Squares of the two other Sidesj becaufe
chis fo belongs to this Sort of Trianglcj that it neceíTarily
foUows ff om its being right-angled.

12. We cali that an accide 7 ital Froperty of a Thing, or
in general an Acádent, which we do not think neceíTary
tp it,- or which fo belóngs to it, thac it might have been
without it, and yet not ceafed to have been what it was :
Thus the Blacknefs in a Tiiangle is an Acádent, becaufe
this Colour is not neceíTary to a Triangle,- and it may
be not Black without ceafingtobe a Triangle.

13. The Produdion of Something which before was
not, we cali Generation-^ thus we fay Pire is generatedj
when we fee Pire where che Wood was before^ fo like-
wife we fay a Chicleen isge»er<?íe 4 , when wefeea Chicken
in che room of anP.gg.

i,|. When a Thing is deflroyed, or ceafes to be what
it was before, we cali it Corruption ;; thus we fay it is a
Corruptiott of the Wood, when we fee the Wood nolon-
ger, but only the Pirein the Place of itj And inthe fame
manner we fay an Egg is corrupted, when we fee the Egg
no longer, but a Chicken in its Place.

15. A Thing is faid to be alteredy when it has under-
gone fome Change, but not fo great a Change as for us
not to know it again, or to have a new Ñame given to
it ; Thus when a Piece of Iron, which was before coid,
is made hot, it is faid to be altered j for this Change is not
lo great, but we know it to be Iron ftill, and do not
give a new Ñame to it. We muft take particular Notice
here, that the Alteration muft make but a modérate
Change ; for if it be fo great, that we cannot know the
Thing thus changed, we do not then fay thac it is alteréd,
but that it is cormpted.

líí. By ú\efirfi Frinciples of natural Things, w'eunder-*
ftand, that which isfirft, and moft limpie in them, or that
of which they are originally compofed, and beyond which
they cannot be reduced. Thus, the firfl Frinciples of a
Chicken, are thoíé Things which are united together to
compofe a Chicken, and which are fo limpie, thac they
themfelves are void of^ Compolition.

G

tf

12 . mat
the accidentAl
Property of a
Thing

1 3. What ií
tneant by the
VP'ord Gene~
ration»

14. What h
meant bythat
of CorruptioTti

Tj*. What h
meant by the
Word Alte^
ratiorik

l 6 i What It ■■
meofit by the
frfi Pr/wcr-
plcs of nalíí*^-
ral Things^

17. Now

RO H AULT’s System

Part íi

-1 That ths
forementioned
Terms fgnify
nOgnore than
iscontained
in the Definid
tíb'n of them.

18, ACanti-
on about the
Meaning of
fome Nontis
SrthJ?anthes.

19. An Er-
rtírtr arijing
from voant of
attending
hercio.

17. Now I dó not precend that the foregoing Definí'
tions contain any fecrec Things in them^ ñor do I defign
they íhould pafs for Things very íublime, as fome Pbilo-
fophers have done,- buton thecontrary, my principal De-
fign in laying chem down here, was no other than to ex-
plain the Meaning of the Terms whi'ch I have defined fo
diílinélly, that no one might be deceived, in putting any
other Senfe upon them more enlarged or reñrained ; and
to do it in fuch a manner, that no Fidtions might be
made out of them.

18. I fhall hefe add one Caution about Words, and it
is this, That though thofe which we cali Nouns fihftantive
were invented to fignify Subftances,- and Adjeóiives and
Verbs properly lignify only Qualities or Modes, or Man-
ners of exiíiiug or adingj yet there are a great many
VVords, which in Grammarpals for Nouns fubftantive;
whofe Signification is the fame as that of Verbs. Thus
when we l'ay that a Walk is wbolefime-i we mean no more
than that it is wholefome to •walk.

19. For want of attending to this Rule, the Generality
of young Men, when they begin to íludy, take the Things
fignified by thefe Sort of Nouns fubftantives, to be real
Beings, and imagine chem to have a particular Exiftence,
and by this Means fill che World wich Scholafiick Entitiés.,
and rational Esitities, which they are many times fo pof-
fefled wich, that they becotne incapa ble, all cheir Lives after,
of applying chemfelves to any Thing that is folid and
iubñaocial.

^ A .$• * * * 4. á, .$> á, A * .%

' c H A P. V.

'The principal Axioms of Natural Philofoph'j.

O

1. TheFmn- AFTER having explained the principal Terms made
datíonof na- ufe of in natural Philofophy; I fhall now lay down
tmaiPhhfi- imporcant which are felf-evident, and which,

being the .Foundation of all PhiloíbphicalTruths, arecon-
fequently the principal Axioms of Philolbphy.
yAxiom I. 2. The firíi is; that Nothrng, or that vjhich has m E»~
ifie-, ice, has no Froperties. Thus we cannot fay that No-
thing is hor, or coid, can be divided, or has Parts, Ó'c.
Therefore where we know there^jj any Property, what-
ever it be, there We mayaffirm, that there is fome Thing,
Rme real Being.

Chap. 5- «/Natural Philosophy. 19

Secondly^ Jt is impojjible that Sotnething Jhould he ?nade
of abfolute Ñothing-^ or that tmre Notbing can hecome -any
Thing. This Axiom is a neceílary Confequence of che
foregoing one, and provesic felF to them who grant tóaA
For if Nothing can be made Something, ic would follow,

.contrary to the preceeding Axiotnj thac Nething has fome
Property : Which is abfurd.

4. When I faid tbaí it is mpojjlhle for Something to he ^.in what
7 nade of Nothing^ I expreflQy added che Word Ahjolute,
becauíe I do not at all doubtj any more than any other any Thing is
Perfonj that a Thing may be made out of what has no- o/ No-
thing of that Thing in it, or to fpeak more clearly, may
be made out of that which is not Chat Thing. Xhus for
Example: No one can doubt, but that Bread may be
made of Water and MeaL which are not yet Bread.

y. Tfoirdly^ No Thmg or Suhflance cari he wholly annihi- sAxhmXll.
lated¡ thatis-i fo ceafe to he, that there fallremain nothing
H dt all of it. Indeed j when any thing wholly difappears,
we eafiily apprehend, that it ceafes to be the Thing that it
was, in ordcrto becomefome new Thing: Thus we ea-
fily apprehendj that Corn ceafes to be Corn, in order to
become Meah and that every Part of the Meal may be
ítill divided into other Parts, fo fmall thac they may be
utterly imperceptible ; but how that which is Something,
can become abfolutely Nothing, this is utterly uncon-
ceivable.

6. Fourthly ^ Every EfféB preftppofes fome Canje. This 6.Axim\V.
is fo generally allowed by all the World, that the dulleft

of all, are led to admire certain EíFeds, for thac very
Reafon, becaufe they are perfwaded that they proceed
from a Caufe, and that this Caufe is wholly unknown to
them. If this was not a very true Axiom, we Ihould not
fo much wonder at that moft known Property of a Load-
ftone for Example j but reft fatisfied, with knowing only
that the Iron does really approach the Loadftone, wichouc
wiíliing for any Thing further.

7. Fifthly, Which is a Confequence of the foregoing 7- xixím v.
Axiom ¡ If voe our felves are not the Caufe of a?iy EffeB, it

muf neceffarily depe?id upon fome other Caufe. Thus, if I
know certainly, thac a particular Eflfedt which is within
. my own felf, does not depend upon me j I certainly con-
clude, that it depends upon fome other Caufe.

8. Sixthly-j Every Thing, as much as it can, endeavours to S...íx¡omVl.
conthiiie in that State Hfwhich it is. Thus, if any Thing

be fquare, it will continué always fquare, and will never
of its own felf become round, orof any other Figure. *rbis

C is

20

10, Axiovt
VIII.

1 1. 77m¿
there <¡re Wrf-
ny T»ore Ax~
ioms.

1 2 . That
7hings are
here treated
of in their
natural
State,

That toe
ought not to
Jay>th<Tt there
ts any Thing
tohich God
cannot do.

ROH AUL’í’ s System PartL

is what others mean, ■when theyfay, thatNothingtendsto
thedeñroyingof icfelf.

9- From whence it follows, Sevejithly, That every Al-
teration is made by femé external Can fe. Thus if we fee a
Flowerin a Carden very freíhin the Morning, and in the
Evening finditwitheredj weconclude, that either the Sun,
orthe Wind, or perhaps fome Perfons roughly bandling of
it, have caufed this Change, and though we could not at
all guefs what it was that had made this Change j yet we
fliould afcribe it to fome Caufe.

10. Eigbthly ; Every Alteration is ahvays proportio-
nahle to the Forcé of the Agent ixihich caufes it. So that
the Thing which is altered continúes, as much as it can,
in its firíl State. Thus if a Body, which moves flowly,
comes upon another Body at reft, and puíhes it before it,
we cannot think that it canmove this latter Body ifwifter
than itgoesitfelf.

11. There are yet vaox&Axioms which I fhall afterwards
draw many Con^sjulions fromj but becaufe they are not
fo general as thefe, I íhall content my felf with men-
cioning them, when I have occafton to make ufe of
them.

12. But before we proceed any furtherj as my De-
fign is to treat of natural Things, and to explain as well
the Caufes by the Effeds, as the EíFedts by the Caufes;
that I may not go beyond the Limits of my Subjedt, but
contain my felf within the Bounds of the Science I treat
of; I exprellly declare, that my Defign is to confider
Things in their ordinary and natural State, and that I pre-
tend not to fay, or determine, what they are, or may be,
inanextraordinaryor preternatural State: Becaufe, I think,
it is great RaiEnefs to undertake to determine, how far
the Power of God can extend it felf, whom I acknow-
ledge to be the Author of every Thing in the World,
and who, I believe, can make a Multitude of Things
above the Capacity of humane Underílanding.

13. Wherefore I will never venture to affirm, that
there is any Thing impoílible with God ; and inftead of
fpeaking in fuch a manner, which is too common amongft
Philofophers, I will contentmy felf, with only faying, that
fuch a Thing is not of the Number of thofc Things which
I know He can do.

s. Satftcrthanitgiiesit¡elf)XSTi- as a(j ^díiido^ of hí» Forcé. Sse
¡efs in be endued with an e/.iJUck bdow, Chap. xi. Art. C.

Forcé, which is to be Hnderílood

Chap. of Natural Philosophy. ¿r

14. And above all Things, I particularJy guard my felf » 4 -
againíi enquiring intothe Myfteries of Faith, andattempc- Z^mZ^nl
ing to explain what is obfcure thereín,- becaufe I am fitím into
firmly perfwaded, that that which God Almighty would -Wí""-
have to be a Myftery to the Ignorant and Unlearned, he '
would have to be fo’likewife to the tnoíl exalted Genius,
and to them who think themfelves much greater Philo- ♦

fophers than I am.

VSts)

C H A P. VI.

Of the Principies of Natural ‘Things.

I N order to know what the Principies are, of which ,, 0/ Mai~
natural Things are compofed, we may take one par- ier.
ticular EíFedl: for a Rule, and examine that-, as for Exaní.-
ple, what is done, when the Wood is converted into Fire :

For by this Means, it will be eafy to judge, what palles in
• other Produdtions of Nature ; and this will, as it were,
lead US by the Hand, and help us to difcover what natu-
ral Frinciples are, and howmany there areof them. Firft
then; becaufe, accordingco the Maxims before eftablifli-
ed, it is imponible to conceive the Wood to be wholly
annihilated, or the Fire to be made out of abfolute No-
thing, therefore we muft think, that there is Something
which before belonged to the Wood, which now belongs
to the Fire, and is therefore common to them both. Now
this, whateyer it be, that fubliíls under thefe twoForms,
we cúlMatter, as others cali itj fo that Maiteria one of
the Principies of natural Things.

2. Semzdly, We apprehend alfo, that there muft necef- í.ofForm,
farily be fomething elle added to Matter, which makes it
to be Wood and not Fire, or to be Fire and not' Wood ;
and whatever this be, which does not caufe Matter to
exift, buc only to exift in that manner, we cali it the
Farm j and this v/e reckon another Principie of natural
Things.

3. yírifotk ohkrved, that though a Thing could not ^■ThatPri.
be made abfolutely out of Nothing, it might however be
made out of what v/as not that Thing. Thus a Chicken Gmeratimof
may be made out of yfkt which is not now a Chicken i ^
fo that the Non-exiftence of a Thing, which he calis
ffivfltm, muft immediately precede the Gpneratjon of

.G-3 if.

22

4.7%4f Tri-
vatíon ought
not tú be cal'
leda Princi'-

^ That then
Afe only exea
Frinclplest
viz. Matttr
and Form,

(j, Tbat itls
necejfary
righíly ío wi~
derjland
•po'oat l^latter
andForm are»

ROHJULT^s SVsTEM Partí,

it; From whence he concludes, thac there are three
Principies of natural Things, Privatiofh Maíter, and
Porm.

4 . But by making Trivatmi a Principie, the Word
Principie becomes ambiguous, and quite another Mean-
ing is given to ir, than when we faid of Matter and Fam,
thac they were the Principies of natural Things ; for it is
certain, that Privation is not at all a Thing, ñor does it go
to the Compoíition of any Thing.

y. Befides there is noReafon to malee a particular Myf-
tery of this Word Priwriaz?; for there is no Body but
knows what it means; and fince it is of no ufe to explain
natural Things by, we conclude, thac 'there are but two
.Principies of natural Things, vía. Matter., and Pmm.

6. But we have not yet raade any great Advances in
the Knowledge of the Things of Nature : For, Heisve-
ry far from underñanding the Nature of Fire, who knows
only thus much, that Matter is neceíTary to the Compo-
fition of it, chatis, it has fomething, we know not what,
in comtnonwichocher Things,- and chata Porm isalfone-
ceíTary toir,- chatis, another Someching, we know not what,
which gives that particular Exiftence to‘ the Fire,- for, as
was obferved befóte, a Thing that is unknown, does not
become known, by giving a Ñame to it ,- we muft there-
fore confider more diftinály, what Matter and Porm par-
ticularly are. We will begin wich Matter, and try to find
oucwhacítois, which we cali we don’t know what, which
is common to all the Things in Nature.

C H A P. VIL

Of Matter,

I. The Me- C I N C E there are but three Particulars neceíTary to a
thed of fmd- O perfed Underftanding of any Thing, viz.. its Ppnce,
its Properties, , and its Accidents-, that we may comprehend
fully what Matter is, we muft diftindly explain what tha
EJfence of it coníifts in j what the Properties of it are ,- arid
what Accidents it is capable of ,- in order to which, we
have no more to do, but to examine' all that weconceive
any way to belong to material TlUígs, coníidered as ma-
terial, that is, to belong to Matter^ and then exadly tO
diftinguifli its EJfence, from its Properties, and Accidents,
' ■ ’ ' 2. No^

Chap. 7 - o/ Natural Philosophy. 23

2. Now according to this Method, if we confider, tbat The accí-
Shough we do noc perfeftly underfland what Hardnefs, Li-

^Kídityj Heat, Coid, Heavinefs, Ugbtnefs, Tajle, Smell, Sound, Matter.
Light, Colour, Trattfpare?Ky, Opacity, and the lilce, are;
yet we underftand enough of chetn, to know, that they
arenone of them infeparable froip Matter j that is, itmay
exifl: without any of them, (forwe feethat fome materi-
al Things are without Hardnefs, fome without ’Liquidity,
íbme without Heat, and fome without Coid, and fo of thc
reñj) wherefore we fay, that the Effence of Matter does
not confiíl in any of thefe Things, but that thefe are ac-
cidental only,

3. But when we confider ybxxex^&extendedmto'Length, 3 . That nx-

Breadth, and Thkkjiefs ^ as having Parts, and thofe f"*

having fome Figure, and that they are mpe 7 ietrahle, we do Uatur.
not judge in the fame manner of theje, ñor think them

mere Accidents of Matter. For, as to Extenfon-^ it is
g certain, that we cannot feparate the Idea of that, from
any Matter whatfoever ; becaufe if Extenfion does not go
along with ir, we immediately lofe the Idea of Matter, in
the fame manner as the Idea of a Triangle vaniflies, if
we ceafe to have in our Minds the Image of a Figure- ter-
minated by three Lines.

4. As lo the Parts of Matter ; we apprehend them to 4 . to have
belong to it fo neceílarily, that we cannot imagine any
Portion of it fo fmall, be it the fmalleft we can conceive,^"

but that if it be upon a plain Superficies, we muft
think at the fame time, that it touches it in one
Parr, and does not touch it in another; that is, this fmall
Portion of Matter, confifts of Parts.

5. With refpcít to Figures-, though it be nothing elle j. ThatF¡~
but the Difpofition of the extreme Parts of a Body, and'S”?,"”"/
perhaps we cannot determine the particular figure or a Matter.
particular Body ; it is bowever manifeñ, that we cannot
conceive any Body, be it ever fo greac, or ever fo fmall,

but at, the fame time we conceive it to have fome
Figure.

6 . Laflly, With regard to Impenetrahility ; lince a cer- That im-
tain Portion of Matter, fuppofe a cuhic Foot, has all that

is neceíTary to fuch a Magnitude, we cannot conceive how cUent «/
aiíother cuhic Foot can be added to it, without making
two cuhic Feet ; For fuppofe any one would reduce them
to one cuhic Foot by Penetration, this would not be fo
much reducing then7jj||to one cuhic Foot, as it would be
deftroying the firft Suppofition ; whence we are led to
think, that the Parts of Matter are in their own Nature
impenetrable. C ^ 7- Now

24 RO H 4 ULT*s System Partí,

7 . ofthe Ef- y, N'ow this being fo i we muft fayj that Exte7ifi(m, Pi-
fnücs o/' ’Vífibility, Figure, and ImpenetrahilHy, are, at leaft, ejfential
ídMtir. Froperties of Matter, becaufe they always go along with

ir, an4 cannot be feparated from it j and thefe being all
that we conceive to belong to Matter neceflárily, for we
know of nothing more, ^we are aíTured, that the EJJence
of Matter coníiíís in one of thefe.

8. Whixt the 8. And becaufe we conceive Exte7iJ¡on before the other
Witter fen- ^hrce, and becaufe we cannot conceive the other Three,
fjis in. without firfl: fuppofing Extenfion, « we ought to think

that ExteTiJíOTi is that in which the Eífence pf Matter
coníiíís.

9. r» mhata 9. If it íliould be here objedted ; That God could make

natural Ph¡- Something to bc the E/7?»fe of Matter, which neither wc,
toacknon- any Man hving, can underltand what it is, we can
ledgetheEf- make no other Anfwer, but only this,- that God, being
je’ni’iaTprof' ^ Things, might create them according to his

fcrties ef own Will j for we do not pretend to determine by our y)

which Reafon cannot come at. Wherefore
■ leaving fuch Sort of Queftions to be treated of by thofe,
who are of a higher Profeffion than that of mere natural
Philofophy, and who carry their Views far beyond what
Reafon can do j we íhall contain our felves within the
Limits which that prefcribes, without invading the Tef-
ritories of others; and conclude from that Knowledge
which wehaveby Reafon, that the EjJénce oí Matter con-
íiíls in Exteiijion, becaufe that is what we firfl: perceive
init, and from which every Propertyof Matfer is derived,
and upon which it depends.

I. We ought to think Extenjz-
en, 8 cc.) It does no more íeem ro
íollow from henee, that, becaufe we
conceive Exrenfion béfore any other
Properties of Matter, and thac rhofe
Properties can'c be conceived to ex-
iíl, without firñ conceiviíig Exceníi-
on,‘ iherefore Extenjion is che Eíl
fenceof Matter,' thanitfolIowsfroHi
henee, that Exijlence isconceived be-
fóte all other Properties of Matter,
and iherefore Exijience isthe Eífence
of Maccer. But fince Excenfion is a
more general Word, and compre-
hends more under ic cban material
Things, itfhouldfeem, thdithatim^
penetrable Solidity which belongs ro
all Matter, ahd to Matter only, and
from which all ics Prooerties maní-

felllyflow, may be more truly calJed
the Eflénce of Matter. '

But furtber j if Extenjion were the
Eflence of Matter, and íb Matter.
theíameas Space it felf ic would
follow, that Matter is infinite, and
necefiarily' eternal, and could nei-
ther have been created, ñor be redu-
ced 10 nothing ; which is very ab-
fiird. Befide 5 ic evidently appeafs
from Gravity, as íhall be afterwards
explained, and from the Motion of
Comees, and from the Vibraciona
of Pendulums, that Space it felf is
not Matter* Wherefore not Exten^
JioTJi but folíd Extenjio'í, impenetra-
ble., which is endued with<a Power of
rejífiing, may (as was before láid) be
more^lj^caUed úiQEJfence of Matter.

10. Fur-

Chap. 7 * Natural Philosophv. 25

10. Further, that we may carry our Knowledge as far ^•^■ThatEx-
as the Light oí Natura will permit j lee us coníider tbat TmreMoT.
the Idea of Extenfion is fo. far from depending upon any

created Thing, that we can fcarce get it out of our
Minds, when we try to imagine Nothing, which we be-
lieve was before the Creation of the World ; which
Ihows that it does not depend upon created Things, that
it is not a Confequence ñor a Property of them^ much
lels is it an Accident or Mode of exifling, but a true
Subílance.

11. It is generally believed, that this is very different ly-Thatth'í
from the Opinión of Arífioíle, becaufe he fays in his Me-
taphyíicksj tbat Matter is not a Thing that can any way the greateji
anfwer to Queftions which relate to EJJence, ^antity,

and indeed, that it is not a certain determináte ”h°emfeíves
Thing j This the Ariftoteliam, for the moíl part, fo in- the Difd^ies
terpretj that they would have us think that Matter is not
Ü ae all extended, ñor has any Exiílence.

12. But Arífiotle feems in this Place to Ipeak of Mat- TjtaAis
ter in general ; for he exprefly diftinguifhes between Ex- comraryteil
tenjion and ^uantity, as every one ought, becaufe we

can conceive the one wichout the other. Thus, for Ex-
ample, a Surveyor of Land conceives at firfl: Sight, that
a Field is extended, but he does not know the Quantity
of it, till after he has meafured it. Now in this Seníe of
the Word Matter, there is no ineonfifteney in faying,
that it may be extended, and yet not be any Thing that
will anfwer to thofe Queftions which Arifiotk there euu-
merates j for thofe Queftions are tp be underftood only of
Matter under fome particular Form ; Thus we cannot
íáy of Matter in general, that it is Hot or Coid, that it
coritains a certain Number of Feet, or that it is fuch a
particular Thing, as Gold, or Wood, or Marble; any
more than we can fay of an Animal in general, that
jt is a Horfe, and not a Dog, or any other particular
Species.

But be this as it will, if Arifiotle was not of this 13
Opinión,' as many of his Interpreters think he was not,-
vye fhall make noDiíBcuIty in this Matter, to^difter from/»», whkh
him ; becaufe we do not govern our felves by Authority, ,

L when we erideavour to eftablilh Things upon Reafon. xuah.

And there feems to me no Reafon to fay, that Matter,
which is the common Subjeói of all Things, has it felf no
Exiftence j for there Difterence betwixt Non-Ex-
iftence and Nothing, or having no Properties.

14. Some

20 ROHAULT^s System Partí.

Some Arifiotelians, who may be fatisíied with this
^LengihT Anfwcr, will perhaps fiad faulc with mCj, becaufe I cali
•Breadth, and. Extenfiofi in Length. Breadth and Thicknefs, a Sub-
^tnmt^be a ^ Mode or Accidcnt. as they do.

Mude'. Thus, for Example, when we ípeak of the Extenfion of
a Table. they underftand that the Extenfion is a Mode.
and the Table the Subflance of ir. But it is ealy to make
appear, that this is a MIftabe arifing from the Manner of
Speaking. and is altogether as grols. as it would be. in fpeak-
ing of the City of 'Rome, to imagine, that thefe were two
differentThings, one the Mode, and theother the Subflance.
But to clear all Difficulty in this Matter. we muft ob-
ferve. that it is of the Nature of a Suhfiance to he able to
exifi •without its Mode ^ on tfie other hand. The Nature of
a Mode is, fiot to be able to exifi without that Stihfiasice of
which it is the Mode. For it is evident, i that the whole
Extenfion of the Table can fubfift without being a Table.
but on the contrary, there can be no Table without Ex-
tenfion. Wherefore. fo far ought we to be from faying,
that Extenfion isa Modeof which the Table is the Sub-
ftance. that we ought to fay. on the contrary, that Ex-
tenfion is the Subflance, and the Form of the Table
the Mode.

15-. mmc Lafily, They who deny Extenfion tobe the Eílénce
tura! Fhihfi- of Mattcr i cannot diflinótly tell us whaf they mean by
fhy has htm Matter, uor in what its Eílénce confifls j and they lay
barren " ^^^wn fo obfcure a Thing for a Principie, that it is im-
pofiSble to draw any Confequence from it. that can en-
lighten our Minds, or ferve to clear up any Truth.
Wherefore we need not be furprized, that their Philo-
íbphy is ib barren, and that it is not capable of explain-
ing the fmallefl Effeót in Nature. Let us now fee if the
fame may be affirmed of the Principie which we have
maintained.

t, That the rohole Extenfion of the
TalfUiScc ) TeSf if neichcr the
/lür the Matter it felf, or Sitbjiance of
the Table exiíled. This Iníiance
rherefore does not prove» that Exten-
Éon is that Subñance or Matter of
íhe Table, but that there mufí neceí-

farily be fome Sttbfiame fubíiñing unr
der the Form of the Tablea which is ic
felf extended i which extended Sub-
ílance is not ExtenGon it íélf, but
fubfifts. in ExcenGon or- extended
Space.

Vry^

CHAR

27

Chap. 8. cf Natural Philosophy.

CHAP. VIII.

So?ne Cor ollar íes of the foregoingNotion.

F R OM what we have now laid down concerniug the ^

EíTence of Matter, we infer in the firíl place, ' thaí imps¡fMe
■mbat the Fhilofophers cali a Vacuutn cannot pojfthly be : For f
by a Vaamm they mean a Space void of all Matter j but phUofipherí
by Space (or Extenfion) we mean the fame Thing as cali a vacu*
Matter j and co ask if there can be any Space without

L

T, That what Vbilofophers cali a
Vacuüm, &C.') This is confiftendy e-
nough faid by him, who aífirms the
EíTence of Matcer co be ExcenCon :
Buc ic is very evidenc from Gravitya
( which íhall afrerwards be briefiy ex-
plained) chac chere muft noc oniy be
a Vacnnm in Narure» buc cbac it is
che far greacefl: Pare.

BefideSj a Vacimm, as I íaid now,
is demonñrated from cheMocionoi
Comets* For fince the Comets are car-
ried with a cmtmual Motion thrmgh
the Heavenly Spaces^from every Part,
and all Ways, and lo all Parts (in
OrMcs which cuc the Orbiis of the
Planees tranfverfeJy every way) tt is
evidentfrom thencCi that the Heavenly
SpaceSi mítji be void of any fenjible
ReJiJiamCi and confeqnently of any
fenfible Matter^ Newc. Opcic: p. 3 lo.
See alfo ihe Noces on Pare II. chap.
2y, 26.

This is ftill further evidenc from
the l'ibrations of Fendu/nrnsi for they
mece with no Reííñance in Spaces,
ouc of which' che Air is exhauíted j
wherefore it is plam, here is no fen-
íible Matter in thoíe Spaces, ñor in
che occulc Pores of the-Bodies them-
’íelves. The Fiflion of Cartes, that
the Smalnefs of his íúbtil Matter ís che
Keafon v/hy the Refiflanceds infenfi-
ble, for a fmall Body ftriking againñ
a large one, cannot move ic in the
leaíf, ñor hinder its Motion, but is
refleíied wiih che Whole of its own
Motion ; this is very weak, and con-
trary both co Keafon and.Experience,
For the famous Sir Tfaac Newton has
demonñrated, chacche Denfity o^n-
id Médiums is pretty nearly vf 0 opcT-
tion to their Reíiííance (Opc. p. 3 1 1 .)
and that they are very much miña-
keHi who íhink that the Rejijianse of

proj eflile "Bodies is infinitely dlminijh-
edj by the infinite Divifion of the
Parts ofthe Fluid i ( Princip. Book II,
Prop. 38. Corol. 2.) For on the con-
trary^ it is evident^ that the Pefifi-
ance tan be but a very little diminifh-
edi by the Divifion of the Parts of the
Fluid (Ibid. Prop. 40. Corol. 3.) For*
the refifihig Fortes of all Fluids are
very nearly as their Denfities, For why
íhould noc che fame Quancicy of Mar-
csCí make che fame Reíiftance, whe-
cher ic bedivided into a greac many
very fmall Parts, or into a few large
ones? 'Wherefore, if chere were no
Vacnitmi ir would follow, that a Bo-
dy moved in Air, or in a Place out
of v/hich the Airisexhauñed, would
meet with as much Dífiiculty, as if
ic were moved in Quick-filver 5
which isconrrary to Experience, and
cherefore ic is evidenc, chac there is a
Vainnm in Nacure, and (as wasíáid be-
fore) thacicis mnch che greaceftParc.

Since cherefore che Eííence of
Maccer does not confuí in Extenfiont
buc in impenetrable SoUdityp wemuñ
íay, chac che whole World is made
up of folid lioddes which move in 3
yatuíim. And we need noc fear, chac
the Phsnomenaof Nacure íhould noc
befo well explained thereby j for che
Explicación of choíe Phíenomena
which feem chiefly co depend upoa
a Plennm» viz, The Barometers^ the
Flux fmd Refinx of the Sea^ the Mo-
tion of the Stars andof Lightt theíe
can be more eaíily and fully explain-
ed upon ocher Principies (as, íhall be
Ihownhereafcerj) buc as cothe ocher
Ph^nomena of Nacure, which de-
pend upon Cauíesnoc fo general, che
Explicaiion of chem is che íame in
our" Syílemas in tbacof Canes.

MattWj

z8 ROHJULT*s Sv^rzM Partí.

Matter, is the fameas toask,if therecan beany Matter with-
ou: Matter, which is amanifeft Contradidtion. And it fig-
nilies nothing to fay, that we can conceive a Space, in
which we fuppofe there is no Light, Colour, Hardneís,
Heat, Weight, in a Word, in which we fuppofe there is
not any one Quality that we can imagine ^ for when
this is done, and all thefe Things denied of Extenfion,
ic is the Accidents only that are taken away from the
Thing, wholé real Effence is at the fame time fuppofed.

2. WhiH thí 2. And here we lEall not trouble our felves to give an
Cmfeqtunce Anfwcr to any one who fhould put the followíng Queftion
cTd'jhc^í to US ; Whecher God could not by his Omnipotence
annihUatethe malte a Vacuum, by annihilating all the Air in a Room,
Boom'” ” hindring any more from coming in ¡ts Place? For, as
we faid before, it does not belong to us determine how
far the Power of God can extend it felf. But if che Que-
ílion be a little altered, and we be only asked, what we
conceive would folio w, if God fhould annihilate all the Air
inaRoom, and not fuffer any other toenter inits Place?
Wefliould return for Anfwer, (not concerning our felves
with what would come to país without the Room,) that
the Walls would approach one another fo near, that there
would remain no Space betwixt them.

3. That the 3 ’ 't may be urged by fome, that the Walls of

vifpfition 0/ a Room exifl: independent of what is contained between
iheWaih ia them, and confequently that they might continué in the
^Roerñfdtpefid ^tatc they were, without approaching one another, though
apan^heEx- vjhsí is becweeo them were annihilated. To which I
that it is very true, that the Exiífence of the
is contained Walls does not depend upon what is contained between
between them, themj but the State they are in, or the Difpofition of
them, in order to compofe a Room, this depends upon
Extenfion, or fome Matter which is between them, and
confequently, this Extenfion cannot be deftroyed withr
ouc deftroying the Difpofition which the Walls wete in
before, though not the Walls themfelves.

4- T-f'ijt is Setondly-, We are to underftand that internal Place,
or the Space vjhkh any Body pojpjfes, » does not at all dijfer
from the Body it felf. And therefore when we fay a
Body changes its Place, we mean its external Place, that
is, viith regard to the Superficies of other Bodies voith <which
it is ftrrounded, to the different Parts of vihich, it may be
dijfereñtly applied.

T. Does noi gt all &c.)

This Indeedis noc true j butic makes
no díHerence as :o the Explicación

of fheVhffinomena of Naiure. Foc
the true De&nition of Place. See
ihe Notes on Qha$. x. Art. 2.

f Thirdly,

Cháp. §. of Natural Philosophy. 29

5. Thirdly, When a Body appears to take up more r- Hoa 3»-
Room than ic did before, withouc our perceiving any Mat-

ter to be added to itj which is what we cali Rarefadion, denfeT.
we conclude i that íbme very fubtile Matter has en-
tered into it, and diítended ics Parts. So likewiíe, when
a Body appears to take up lefs Room than it did before,
withouc our perceiving any thing to be taken from it:,
which is what we cali Conienfation^ it is our Opinión,
that forae imperceptible Matter is gone out of its Pores,
and that by this means its Parts approacb nearer to each
other. For fince Extenfion and Matter are to us the fame
Thing j we cannot conceive that a Body íhould appear
more or lels extended, lee the Manner be what it will,
but that it muft have more or lefs Matter.

6 . And this does not hinder, but thatwe maylay with S'

Ariflotle, that a rare Body is that, which has but a little
Matter, and poíTeíTes a large Space, and a denfe Body, is thM a Lre-
that which poffeíles a fmall Space, and has a great dcal fy“!

of Matter ; or, which is the fame Thing, that a rarefy’d a

Body does not acquire any new Matter, ñor a conden- emímfed-Bo-
fed Body lofe any of its own. For this imperceptible
Matter which we fpeak of, ought to be confidered as a
Thing that is foreign, and which does not at all belong to
the Body it enters into, or comes out of, when it is, ra-
refyed or condenfed. Thus when Pafte is turned into
Bread, it is rarefyed before, and while it is baking, yet
we don’t iay, becaufe of this, that we have more Bread
than we had Pañe^ though ic is vilible, that a great deal
of Air is got into thofe large Spaces which we cali the
Eyes of the Bread-, becaufe, what is thus got in, is not
what we cali Bread ; So alfo when we preís the Crumb
of the Bread in our Hand, and bring ic to a lefs Com-
pafsj though we are fure that a great deal of Air is
^ueezed out of it, yet we don’t fay that there is lefs
Crumb than there was before ; becaufe there remains yet
all that we cali Crumb, and the Air which went out of
it, did not belong to it.

7. What we have now faid about Rarefañion, may be 7. mmtt
thought perhaps hardly to agree with what we experi-

cnce in a Chefnut, which, when puc upon the Fire, hmjis
buríls with a Noifej for it may perhaps be imagined,

I, Jhat Jome very fnbtileMatteri
&c.) When any Body is rarefvedj
ic is often very manifell,

Parts are diíiended by the Eñírance
of the Air, or íbme more fubtile
Matter. ' Buc this does not follow

from a but either from the

Liquidnefs, or from an elaíHck Forcei
or from Gravicy and Preííüres or
from íbme accidental Mótion in that
fubtile Macter which enters inttí che
Pores of thQ rarefyed Ecdy.

that

20

2. That the
Vf'''orlii is in-
dejinite»

That it is
impojjible
that thern
Jhonld bema-
ny VVorlds,

10. That the
Matter of the
Heavetis,and
of the Hodies
sipón thss
Zarthy are of
the fame
Kfnd»

RO HJULT’s System Partí,

that the fubtile Matter whid: enters through the Pores
of the Husk of the Chefnut, may come out with the fame
cafe as it enters in, without breaking, or making any
Noife. 'Buc this Difficulty ís eafily refolved, if we con-
fiderj that it is not the foreign Matter that enters in, and
comes out of the Chefnutj which is the immediate Caufe
of the Noife j but the more groís Parts of the Chefnut
it felf, which are torn in Pieces, and put in fuch Motion, » by
the fubtile Matter which enters the Pores like ib many
little Wedgesj that they break the Husk with a Noife.

8. Fourthly-, Weconcludej ^ that the World isindefinite-¡
becaufe at how great diftance foever we fet its Bounds, it
is impoffible for us not to imagine Exteníion to be ftilf
beyond. Now Exteníion and Matterj being, as was faid
before, the fame Thing j we have no Notion of the
World’s being fo big, but we can imagine it to be ílill
bigger.

p. Fifthly, It is evidente that though we can fee no
Reafon why there may not be many Bodies like to our
Earth, and capable of containing many Animáis^ as that
does ; yet it is impoffible ? that there fiould be many
Worlds j for thisj in which we arcj poíIéíTes all that Space
which we are a ble to conceive.

10. Sixthly, Becaufe the Idea we have of theExtenfion
of the Heavens is che fame as that of the Exteníion of
Things here below, we ought to think 4 that they are of
ths fame Ktndj and it is no Objedion againfi: chis, to fay,

’ that the Exteníion or Matter of the Heavens is brighter,
and not fo mutable as that of Things here below, becaufe
this Difference regards only the Accidenta of Matter, and
not the EJfence of it.

1, "By the ftthiile Matter, Scc>) Or
rather by the included Air, which is
very much rarefyed by che Heac, and
cears the Chefnut in pieces.

2,. 7 hat the Ví^'orld is indcfnitey
Src.) From che Hypocheíis of a Pie-
nnnii it muft neceííaríiy follow, that.
che World is really zx\¿\.xu\y ¡>fmite,
nay, that it is uncreacedand eiernal,
(as was faid before.) Buc fince ic is
evldenti that Exteníion may exili:
wichoiic Matter ; whecher che mace-
riai World be infinite or no, fuch is
the ShoTtnels of humane Underíland-
ing. that ic cannot certainiy ba

known í Therefore it may very well
be called indefinlce ftill.

3. That there Jhotild be many
Vf'orlds, &c.) Ic is evi-denc, thac
there may be many Earths like this
Globeof ours j that rhare may alió be
many Syílcms of Stars and Planets
difperfed through the vaft Immen-
fuy of Spacei buc whether there be a
P ¡en Jim or no, the xohole Univerfct
which may properly be called the

l, can of NeceíBty be but one.

4. That they are of the Jame Kfndt
&c.) This isequally míe, whacever
be che EíTence of Matter.

1 1 , Lafly

Chap. 9- Natural Philosophy. 31:

II. Laftly, We cannot aíHrm, that a Veílél filled with ” f»»

Lead ' conrains more Matter than if ít were filled with
Wax, ihough ic be heavier ; for Heavinefs is not eíTential (¡uai^iamity
to Matter, but only Extenfion, which we íuppofc to be Matter.
equal in them both.

13. ThacNotion alone which we bave eftablilhed con-
cerning the EíTence of Matter, has been the only Princi- MTtlrZay
pie we have made ufe of, to anfwer all the foregoing Que- rnake a díj-
ñions with fo much Eafe; whence there is Room to be-
Heve, that we may with the fame Eafe give a fatisfadtory Áuths.
Anfwer to many more, if we reafon in the fame manner
about any of its Froperties : The firft that oíFers it felf is
Divifihility, which is the more copious, becaufeallits Va-
riety of Figures depend upon it.

i.Cmtamsmore Matter, &c. This i come to dilcourfé of the Nature of
is abfoiucely falfej as íhall be ftilly I Gravicy.
demonftraced afcerwards, when we \

CHAP. IX.

Of the Divijibilit'j of Matter,

W HEN we confider a determinare Portion of Mat-
ter without Prejudice, and compare it with other
Portions of Matter with whicb it is encompaíTed, we ea-
fily conceive that its particular Exiftence is wholly inde-
pendent of thofe that are near it, and that ic does noc
ceaíé to be whac it is, by being joined or united to other
Portions of Matter j the firft Portion of Matter there-
fore is feparable from thofe with which ic is united ; and
this fhows the Divifibility of Matter ; and the Poffibility
ef having its Parts divided into ftill leffer Particles.

2. Indeedj when we confider the Power of God, and p¡ Epíco-
his abfolute Dominion over all Things that are in the
World, we cannot doubt, but that he is able to make are i'eally di~
certain Parts of Matter of fuch a Nature, that there is no
Being in the Univerfe capable of dividing them ,• whence ic
would follow, that chefe Parts would not at all differ from
thofe little Bodies, which Epicurus calis Atoms : But this
►Property of not being capable of being* divided by any
external Being, is arbitrary, and not builc upon any na-
tural Principie, but onlv*on a mereSuppofition, which
does not alter cheir reaf^ature,' and therefore we may, ^
notwichftanding this, hold it for certain, that all Matter

§2

g , That Mat-
ter is divift-
blt in all
Pomts that
can be afpgn-
ci»

That the
jTHrfiber of
Pomts affign-
ahle in Mal-
ttTi isindifi^
nitCi and that
Matter is in-.
definitely di-
vífible,

Tab.I, Fig.i.

ROHJULT>s System Partí

<))

is diviíible. The whole Difficulty in this Matter is, how
many Parts a certain Portion of Matter can be divided
into.

3. In order to folve this Difficulty, we muft rémem-
ber, Chat all the Variety that we can conceive to be in
Matter, arifes from che Forms which diftinguiffi its Parts
from each other ; for of itfelf ic is peffedrly homogene-
ous, that is, all alike, being only a Subftance extended
intoLength, Breadth, and Thicknels; whereforewe can-
noc buc think, that whatever it is capable of iri one
Pare, ic is alfo capable of in all othér Parts. As there-
fore we cannot doubt but that it is diviíible in fome
Poines, fo alfo is it diviíible in all the Points that can be
affigned.

4. Now that the Number or Points which we can con-
ceive in a decerminate Quautity of Matter {an ineh for
Example) is indefinite; there are many Demonñrations
in Geometry to Ihow, one of which I ffiall give, which
feems to me very eafy. Let two indefinite Lines AB, CD,
be drawn parallel to each other, and at an Inch diftance ;
then the Line E F, which is perpendicular to them, and
limited by them, will be alio an Inch long. Then let
the Point A, in the Line A B, be taken on the left
Hand of the Line E F, and, if you will, at an Inch
diftance from ir, on the Line CD to the right Hand of
E F, let as many Points G, H, D, ó’c. as you pleaíé be
taken, and at any diftance from each other j to which
let as many ñreight Lines be drawn from Aj as AG,
AH, AD. Then it is evident,- that the Line AG will
país through the Point I of the Line E F ; that the Line
A H will país through the Point L which is higher j and the
Line A D will país through the Point M which* is higher
ftill; and fo on j and becaufe the Line CD is indefinite,
and an indefinite Number of Points, fuch asG,H,Dmay
be taken upon ic, ic will follow, that Lines drawn from A
to all thofe Points, will mark an indefinite Number of
Points on the Line E F different from each other, and
which approach nearer and nearer to the Extremity E,
withour any one of them ever paíling through the Point
E, becaufe the Line CD is fuppofed tobe parallel to AB.
Wherefore, (becaufe the Length of EF was taken atplea- ,1
fure, and the íáme Demonftration holds for any other tal);
Length whatfoever,-) we muft acknowledge, that an in-
definita Number of Points mc’ be affigned in any de-
termínate Portion of Matter, am' confequently that Mat-
ter is indefinitdy divifible.

Chap. 9- of Natural Philosophy. 53

5. This Truch may alio be de.monftrated frotn this Con- s-^nuhcr _
lideration, thac there are fome Quantities that are incom-
menfurable, thac is, have no common Meafure. Thus,
íuppofe A'BCD co be a Square, it may be geometrically Tab.i.ríg.i.
demonñrated, that che Side AB, is incommenlurable co che
Diagonal A C. Lee us then imagine in our Minds che

Line A B, which is an ineh long fuppofcj co be divided
inco a hundred Thoufand equal Parts, and every one of
thefe into a hundred Thoufand other Parts thac are equal
alfo, andagain, every one of thefe into a Hundred Thoufand
other Pares equal to one anocher flill ; we may go on in che
Diviíion Chus, for an Age togecher, withouc ever being
able to come at Parts fo fmall, as to fay, thac the Line
AC contains a certain determinate Number of them and -
no more. Now this could not be fo, if Exteníion were
not indefinitely diviíible,- for then afeer we had divided
the Line AB, for inflance, into as many Parts as it is
^ poflible for Extenfion to be divided into, the Line AC
would neceííarily contain ‘ a certain determinate Num-
ber of thofe Parts. We muíl: therefore conclude, thac .f"'
every Thing which is extended, and every Portion of * h,
Macter, is indefinitely diviíible.

6 . This Conclufion of Arifiotle's, hath been aííénted to g. An oí-
by all his Foll^'ers, except a very fewj and they depart-

ed from it onl* becauíc they thoughc they contradidled
themfelves : For, fay they, if two Bodies be íuppofed un-
equal, andif they can be divided indefinitely, it will follow,
thac the Number of Parts of which the one is compo-
fed, is equal co the Number of Parts of which the other
is compofed ; and from thence it will follow, that
they are both equal, which is contrary co the firft Sup-
poíition.

7. But here is a double Miñake. Firft, they did not 7. An An-
coníider, thac Equality and Inequalicy are Properties oí

finiré Things, which can be comprehended and compared
together by humane Underftanding; but they cannot be
applied to indefinite Quantities which humane Under-
ftanding cannot comprehend or compare together, any
more than it can a Body with a Superficies, or a Super-
ficies wich a Line. Bur, if it could be faid, thatof two
unequal Bodies, divided in the foregoing Manner, as the

1. A certain determinate Kam-
ber, Scc,) For it the
Tab. I. AB could be divided/l^lRo
2* thoíeímalleíl Pares, che Line
ACi and al] other Lines

could be divided alió inco them 5 fó
tha c one of thofe fmalleíl Pares would
be checommon Meafure of che Lines
AB, AC,- andof all other Lines.

D

Líofl

V

34

RO H AU LT's System Part I»

Line EF was divided, the Number of the Parts in the
One, was equal to the Number of the Parts in the Other j
we could not conclude from thence, t that the two Bo-
dies themfelves were equal ^ becaufethe Parts of the one,
are bigger in Proportion than the Parts of the other;
There is therefore no Contradidtion in chis particular, but
the foregoing Denaonftration holds in its full forcé,
g. ^mthtr 8. Others attack the indefinice Divitibilty of Matter,
objeSion. gnother way ; by faying, that it would from thence fol-
lovv, that a fmall Portion of Matter, fuch as a Cube, a
quarter of an Inch high, might be divided into as many
thin fquare Pieces, as would cover the whole Globe of
the Earth, if it were much bigger than it is j whicb, they
think, is abfurd.

5. yín/mr. 9. But thefe have no more Reafon of their Side than
the other; for their Objedion is founded upon this fingle
Maxim of their own, That every Thing is abfurd, which
our Imagination caiit coinprehend: This is a very grofs Mi* gj,
ftake, and unworthy of a Philofopher, who cannot but know,
that there are an infinite Number of Truths, which it is
certain our Comprehenfion cannot attain to. Many Exam-
ples might be given of this, but I Ihall content my felf
with Two, both which relate to theSubjed we are now
treating of, viz,. The Sheets of Gold made by Gold-
beaters, and the Gold Wire made by W* -drawers.

10. Cmctrn- lo. In order toa clear Conception hereof, wemuftfirft
‘Üfnlf^Go/} appears by Experience, that the Weight of

madfhyGM- an equal Quantity of Gold and Water is as 19 to 2, fo
biaiers. that if a Cubick Foot of Water weighs 71 Pounds,

1, That the two Jodies themfehes
are eqttali &c.) Whac ís laid of
QuancicUs decreaíing infinicely Jicclej
mayalíb be underñood of Quancities
increafmg infinicely great; that is,
Quancities infinicely greac, are not
therefore all equal co eaeh other. For
a Line drawn trom a Point infinite-
ly, oneway, Í 3 buchaifal./lnedrawn
from a Poinc infinicely. two ways.
And a Rectangle of an infinite
Heighth, upon a finice Bafc, may be
y, &c. of a Reccangle of an
infinite Heighi alio, upon a propor*
tionable Bafe. And? in Heterogene-
ous Quancities, an infinite Line, is
nbc only not equal, but is infinicely
lefs than an infini ce . 'superficies i and
an infinite Superficies, than an infi-
nite íblid Space. And in aíolid Space,
a Cylinder infinite in Length, is not

only not equal in Quantity,. but ís
really infinitely lefs, than an infinite
íolid Space of two Limenfions, •viz..
Length and Breadth i andan infinite
íolid Space of two Dimenfions, is
infinitely leísthan an infinite Space of
all the Dimenfions. Whence? by the
way, it appears, how weakly they
argüe, who, becauíe Space (and the
íáme istrueof Duration) may be di-
vided into innumerable Parts which
aie unequal > and in wjiníte Space
(or Duration) the Number of the
greateñ Parts is as much infinite as
that of che leafti vwhich^ they think
abíurd, becauíe they believe all In
finites co be equal in every refpeíl: ;
conclude from henee, that there can
ti^o fuch Thing at all as Infinite
Sp^,{or Duration.)

X a cu-

35

Chap . g . of N-aturáí, PhiloSophy.

‘ a cubickFoocof Gold will weigh 1349 Poundsori 21584
Ounces. 3 Now a cubick Foot contains 2985984 Cu-
bíck Lines, and cherefore4 an Ouncc of Gold contains
1381.ii.fl cubick Lines. Wherefore an Ouncc of Gold,
reduced into the Form of a Cube, will be s very near
5 f Lines high, and its Bafe ^ abouc 26-!} fquare Lines.

This being ío, the next Thing to be known, is, that the
Gold-beaters inakeout of an Ounce of Gold 2730 whole
Leaves of 34 fquare Lines each, beíides what they cali
the Wafte, which is the l'mall Shreds that are cut ofF, and
amount tb almofthalf: The Superficies of 7 every one of
thefe Leaves is iijó Lines fquare, fo that if they were
all placed regularly by one another, they would 8 malte
one Superficies of 3155880 fquare Lines 5 to which if we
add 9 buc a third Part, which is the leaíl that goes into
Shreds, it will folloW) that a Gold-beater makes out of
an Ounce of Gold 4207840 fquare Lines. Now fince
this Superficies m exceeds the Bafeof a Cube of Gold ot
an Ounce weight 159092 times, it is certain, that That
Cube, which, aswas faid befóte, did note.xceed 5,- Lines
inHeight, isdivided into 159092 fquare Leaves.

II. Though this Divifion of Gold be very furprizing, n.TheDi-
yet it is very far fhort of what is done by Wire-drawers.

I have feen feveral Ingots of Silver in the Figure of Cy- ¿ramú.
linders, which, weighed eight Pounds a piece ,• one of
them, which feemed to me more regular than the reft,
was two Foot and eight Inches long, and two Inches and ^

T. xA cubícFaotof Gold, &c.) For

I :

2. Or 21584 Oimces) For i6
Ounces make a French Pouad. See
Prejlet, Nouvd. Elem. Mathemat, 3 .
Edil, í.partjib. 2.

3. Nom a ciihic Foot) The Propor-
ticn between a Line and a Foot, is
as I co 1445 now in chis concinued
^eometricai Proporción, che Number
x$ 2985984: ThereforebecaufeCubes
are in a criplicace Ratioof cheir Sides,

3 ciibic Lineis coa cubic Foot, as j
to 2985984, that is, a cubic Foot
contains Lines,

4. xA» Ounce of Gold) A cubic
FoocofGold, which weighs2C584
Ounces, comains 2985984 cubic

^inc.s,' chereíore by che following
Proporción, ic is, 21584 Ounces:
2985984 cubic Lines : : i Oun^jf.

Ll»"- f(c|r

5. Very near 5Í Lines kigh) For
the Cube Root of i38^J.¿|.J-

D 2

is very nearly ¡\, though j-fisflill
nearer, For the Cube of y| is
•S/f'ff i And the Cube of ,4 is

'3%íT-

6 . ^-iboat f fjaare Lines) For
the Square of sj- is pretty nearly

7. Every one of thefe Leaves) Fot
the Sid^ of a Leaf, was faid before co
be 34 Linesi cheSquare of which is
'il5(J.

8 . Makeone Sn^e'^fcies) Mü]úp]y
1156 che Number of fquare Lines in
one Leat, by 2730 the Number of
Leaves, and it will make 5155880.

9. ‘Bnt a third Fart) To which
Superficies, it we add a third Pare o£
3155880, rhat is, 1051960 ic will
make 4207840.

10. Exceeds theUafe) Thatis,the
Superficies 4207840, contains che
BaíeofrhaíGube, ür26,^J-, 15909Í
times.

nme

RO H AU L'í’ s Si^^sTEM Partí.

nine Lines aboutj fo that ^ tha Cylindrical Superficies
was 12672 fquare Lines. Afcer this Superficies was co-
vered over wich leveral Leaves of Goldj, which all together
weighed half an Ounce j the whole Cylinder was drawn
through Hales made in a Píate of Steel, till it became
fuch as the fmalleñWire that ismade in this City; I took
25 Fathom or 150 Foot of it, and weighed them in an
exad pair of Scales, and found that they weighed but 3<>
Grains, wantingabout of a Grain. Wherefore ^ the
whole Cylinder ought tohave been drawn into a Wire of
307200 Foot long ; Whenceitfollows, 3 that if is 115200'
times longer than it was before, and that its Superficies is
become4 three hund'red and forty times as much. To
which if we add, that when this fmall Wire is made into
a thin Píate, to cover Silkwith, s the Superficies istwice

as

T. 77 ¡e Cylindrical Snperjicics) For
cwo Feet and eighc laches (thac is
384 Lines) which is che Height of
rhe Cylinder, mulciplied by cwo
Inches and nine Lines (that is 33
Lines) which is the Circumference
of che Bafea makes 1x672..

2. The whole Cylinder) Firíl lee
the whole Cylinder (which, as was
íkíd before, was 8 pounds) be redu-
ced into Grains

8 Pounds by 16,
which makes 118
Ounces.

rzS Ounces by S,
which makes 1024
Drachnis.

!024 Drachms by
3 j which makes
3072 Scruples.
jcya Scruplesby 2,
which makes 6144
half Scruples.
6144 half Scruples
by 12) which
makes 73728
^ Grains.

by muldplying^

Then by che foliowíng Proporti-
onj 36 Grains \ lyo Feeí : ; 73728
Grains'. 307200 Ffíí.

3. Thatitis Myxco times longer)
For multiply : Feet and eight Inches
(which is the Lengthof the Cylinder)
or 32 Inches by r i fiQo, anditwil!
make 36S6.400 inehe.s, that i.s,
307200 Feet (che Lengch of che
whole Wire.)

4. Three hundred and forty times as
m¡ch) Lee che whole Cyiinderof Sil-

ver which is co be drawn into Wire,-
be called A, and fuppofe another Cy-
linder B of an equal Baíe, but 1 1 7200
times higber, and let the Cylinder of
W iré be called C. Ic is maaifeíl: chac
the Superbeies of the Cylinder Ba and
the Superficies of che Cylinder A, are
co one another as i lyzoo co i, that
is, as che Heighc of che Cylinder B'
to che Heighc of ihe Cylinder A,
that is, as che ’:.aíe of the Cylinder
A, to the Baíe of the Cylinder B-
(for che Bales of equal Cylinders are
reciprocally as cheirHeíghts)íA/3f íV,as
che Baíe of che Cylinder Ba co the
Bafeot che Cylinder C. Nowifwe
íuppofej according co Cavallerins’s
Doátrineof Indivifibles, that che Su-
perficies of Cylinders confift oLan in-
finite Number ot Circumferences oí
Gíreles equal coche Bales, then the
Superficiesot the Cylinder B, willbe
co che Superficies or the Cylinder C,
as che Circumferences, or as che Ra-
dius’sof cheirBafesi now cheRadi-
us’s are to one another in a fubdupli-
cateRario of che Area’s of che Cir-
cles : If cherefore the Superficies of
che Cylinder B, befuDpofed 117200,
the Superficies of che tiyiinder C will
be a mean Proporcional between
117200 and I (chatis, 340 very
nearJy) and che Superficies of che
Cylinder A will be E . ^ D. '^/
7. The Siiperpcies is twice as big)
^_^che Cylinder be made Bar, íts
Superficies is made into cwo
Para.íélograms, v^hich becauíe chey
lie one upon another, form a chin
.Parallelepipedon, capable of being
made as chin agaln, which is done by
chc'

37

CJiap. 9- ^Natural Philosophy,

as big; fo that it. then is encreafed co fix hundred and
eighry times as much as it was at firíi, 6 and therefore
contains ^616^60 fquare Lines. Now after this-Wire is
madeinto fotliin aPIate^ its Superficies isítill comed all
over with Gold ; fo that only half an Ounce of Gold
with which the Píate is coveredj is made fo thiiij that its
Superficies is ^616^60 Square Lines. 7 Which Super-
ficies exceeds 32579^ times the Bafe of a Cube of Gcld
of an Ounce weight, and twenty fix fquare Lines and ¿a
in Breadthj from whence it folio ws, that the Tfaickneís
of the Gold which the Sjlver Píate is covered with, is not
above tttvvt ^ of Balf the Height or TTjrrr^ of the
whole Height of a Cube of Gold of an Ounce weight ,•
fo-that the Quantity of 5-^ Lines is divided into 651590
equal Parts.

12. If we confider further, that Gold is capable of be- Thefm.
ing divided ftill more, if there were any Occafion for it ; fámns’offhs
and above alh if we confider that what we have now ex- Dhijionof
amined is done by Men, and with Inñruments that are Matíer,teach
very grofs and dull, and that there are in Nature many Tcueljud^t.
Things, which are vaftly more fine and fubtile j we fliall »/ tu
clearly fee, that what exceeds our Imagination, isnotg°^"°-f
therefore impoíEblei and that it is not for us to prefume,

as many do, tofetBoundsto the Power of God.

13. Lafl:ly,<ij^e are carefully to obferve, that That Di-
vifion which we make in our Minds and Imaginations,
malees no Alteration at all in Matter, but that all real mithout
Divifion arifes from Motion ; that is, in order for a Por-

tion oí Matter to be really divided from thát to which itis
United, it muft neceílarily be feparated from it. And
henee it is, that Motion is fo neceíiíary, and the Knowledge
of it fo ufeful, that Ariflotk fays, that he who does not
underftand Motion well, muft neceíTarily be ignorant of
all natural Things.

the Workmen, who beat it as rhin as
they can, fo that the Superficies of
the Cylinder is thereby doubled.

6. And therefore.conrains) Multi-
ply 1167a Superficies ot the
Silvei betore it is beaten, by 6 8o, and
it will make 8 6 1 6960,

^ 7. Which Srtjicrpcics eífceeds) 4 ^

p 3 . CHAP;

vide 8616560 by 16.^' and it wilI
make 317797.

8. Of half the Htight) Becaufé the
Gold with which the Silvcr-Wire is
covered-was only half an Ounce, that
is, half a Cube of Gold of an Oiincs
Weight.

38

I. JVhat it is
ío be moved*

1. IVhat Mo-^
tion and Reji
are*

RO H AUL^’s System Partí,

C H A P. X,

Of Motion and Reft,

B e CAUSE it js eafier to underftand what Motion
is, by JExperience, than to give a Definition of it,
or to find out the Caufe, I fhall hete make ufe of a fa-,
miliar Example, agreed upon by all, which may ferve to
explain to us the Nature of Motion .

I. Suppofe a Man in a ealm Day waiking on Foot in
a Park planted with Trees, and that at the Beginning he
is obferved to be between the firft Trees in the Walk,
and then between the Secoud, and fo to continué on
waiking till he comes at the End j no Body doubts but
the Man thus waiking tnoves, and that every Step he takes
is a real Motion. Confider now, that the Motion qf
this Man is fomething new, which was not in him be-
fóte ,• and then if we take an exa£t Account of what we
conceive to have come to him lince he began to be mo-
ved, and rejed every Thing which we certainly know is
not Motion, we are fure that what remains, is, without
doubt, the Thing we enquire after, and &-ú this will íhow
wherein Motion properly confito.

2 . Now becaufe we do not acknowledge a Vaccuum, as Be-
mocritus and Epkurus did, therefore we cannot fay with
them, that this Man which we are fpeaking of, applies
himfef to different Parts of Space, becaufe we do not ,
diftinguifh Space from Matter as they did ,• wherefore in
the Éxample now mentioned, there are.three Things to
be confidered by us. Firñ, The Defire of Waiking in
the Man; Secondly, The E Fort he makes to put this
Defire in Execution ; And Thirdif,. The Correfpondence,
or the JitcceJJlve Application of the'txternal Parts of this
Man, to tire different Parts of the Bodies which encom7
pafs him, and immediately touch him. No.^y it is evi-
dent,, that the Defire which this Man has, is not' the Mo-
tion of him; for Defire is nothing but Thought, and^^e
acknowledge many Things to be moved, which we do , j
not alio w to have any Thought. Sp likewile we ought noF U.
to thinfc, that the Motion of ¿e Man confito in the Eff
fort which he makes towards W^fkíng : For thougb we
ípay truly fay, that all Bodies vlthich move, have an EF
fort (as we ícnow they fometimes have, thougb they da
KOt moye) yet we are rather tp think, ihat this Effort oí"

thñ

!

39

'Chap.ro, o/ Natural Philosophy.

the Caufe of the Motion^ and not the Motion in /elf.
Norhing therefore remains but that Motion con 0 s in ¡ the
fuccejpve Application of a Bodj to the dijfercnt Parts of
thofe Bodies lohich are immediately ahout it j wbence it
folio wsalfoj ihsx the Refi of a Body, is the continual Applica-
tion of that Body to the fame Parts of thofe Bodies mihich
are ahout it and immediately touch it.

D 4 3. It

I. SíícceJJive j^pplt catión of a *Bq-
dyi &c.) TheDiipute abone ehe Na-
ture and Definición of Motion^ a-
mongft the Wricers of Philofophy,
has always been very perplexed. ^
ruppofe, becaufe, noc fiifificiently 9
cending to the differenc Senfesof an
ambiguous Word, chey endeavoured
tocomprehend that.\n one Defini-
tion, which oughe to have been ve-
ry exadlly difiinguifhed into ics dif-
^ ferenc Parts. That Motion (or ra-
ther the Effeft of Motion) in gene-
ral, is a Tranjlation of a 'Body jrom
one Place to amther, is pretty well
agreed amongft them all. Biu what
is meanc by being tranjlatcd from one
place ío another, here che Contro-
verfy lies, and Philofophers differ
■widely. They who define Motion by
comparingtheThirí^/hichis moved,
not with che Bodies^fe enconipaPsic,
but only wich '•'pace which is im-
movcable and infinite, can never
know or underñand, whether any
Body at all reíts, ñor what the abíb-
luteCelericy of thofe Bodies that are
moved ¡sí forbefides,thacrhis whole
Globe ot che Ear;.h revolves about
the Sun, iccan never be known v/he-
ther or 110 the Center of chis whole
Syflem, in which all the Bodies re-
lating to US is contained, reíls, or is
moved uniformly in a ílreighcLij^
Again, chey who define Mociot^^
comparing theThing whicl^rmo-
ved, noc with infiniteSpace^uc with
oiher Bodies, and choíe ac a very
great Diftance, chefe neceíl'arily make
fome Body jjj^Mark by which all
Mocioi^ to be meafuredj which,

' whyjlwic felf is at reft, or, with ref-
pdt to Bodies ac a íiill gri ater di-
is moved, is impollible to be
PI^KnownUkewife. Laílly, They who
define Motion by comparing
Thing which they fay is moved^g®
with diftant Bodies, but oniy^lKch
that Superficies which immediately
touchesitj icisvery weak in them
to fay, that thofe Thíngs are truly
at relt, which being conneífed with

theParciculars of otber Bodies, are mo-
ved with the greaceft Swiftneísj as
cheGlobeof the Earth which is in-
compafled with Air, and revolves
abone the Sun, And on che contra»
ry i that they only can be faid to be
moved, tljac with the utmoft Forcé,
and Refiíiance which chey can make,
can do no more than barely hinder
therafelves from being. carried along
with other Bodies. as Fiíhes which
ítrive againft theStream.

But if we rightly diñingu'fh the
differenc Senfes of the ambiguous
Word, chis whole Mift will immedi-
ately vani/h. For a Thing in Mo-
ción, may be coníidered in chree
Refpeíis i by comparing i t with the
Parts of infinite and iminoveable
Space.j orwichBo^íCí that furround
it at a difiance, or with that Snper-
ficies which immediately tonchei it,

If chefe three Confideracions be ex-
a 6 ily üiífinguifhed into dieir feyeral
Parts, all fucure Difputes about Mo-
tion will be very eafy. Firfl then,
a Thing in Motion may be compa-
red with the Parts of Space'. And,,
becaufe the Parts of Space are infinite
and itnnioveable, and cannot under-
go any Cbangelike Maccer j therefore
tha^Change of Sicuation, which is
made with refpeífc to che Parts ót
Space, wuhout any regará had to
the Bodies, which encompafsit, may
rightly be called, ahjohuch and trnly
proper Motion. Spcondiy, a Thing
•n Motion may b? enmpared v.’íth
diñant Bodies, and becaufe a Body
may in chis manner be transfer»
red along with other Bodies which
immediately furround it; therefore
Chat Change of Situation which is
made with refpe£l to thofe Bodies
which are at a diftance, and not to
thofe which are near, may properly
be called, relathely common Motion,
Laftly, a Thing in Motion, may be
compared with the Superficiesof thoíü
Bodies wtiich immediately touch it :
A nd becaufe, whatíoever is chus mo-
ved, may poftibly have, no abfolntc
or

40

ROHJUL^’s SysTEií Partí»

j.in ordeno It is to be obfervcd here, thac when we fpeak of

^whlsheTaBo- Motioii or Rcftj we always mean an immedi^e Ápplica-

dy be in Mo- tioH, and havc no Regard to the Relation a Body ftands
ihnor no, in to TMngs ac 3 diñancCj, any further.than to confider
lJIdóf”° 7 ri- fuch fort of Relation as a mere external Denomination
farinnHmth only, -whicli makesno Alteration intheThing, and which

"Bodies at a íc

d’ftatite.

br iommm Motíon ac all (as if an
Arrow were íhotcowards rhe Wefí?
wiih the fame Swiftnefs, that the
Earth turns cowards che Eaft s) and
on the contrary, that which in chis
Tefpeét is ac reft, may really be
trañsferred with both ahfolute and
cammon Motíon (asBodieshldin the
Bowels ot the Earth) therefore thac
Change of Sicuation which is made
v/ich refpedl to thofe Superficies,
which immediacely couch che Thing
moved, may righdy be calied
rtlaúvelj proper »

Firíi, Abjhlutely and trnly proper
Motíon, is the Appücatitn of a Body
to the dijfcrent parís of infinite and
inmioveabte Space, And chis is in-
deed alone abfoluce and proper Mo-
ción, which is always generaced and
changed byche Forces imprefled up-
en the Body tha.c^ is moved, and by
them onlyj and to which alone are
owing the real Forces of all Bctdies to
move ocher Bodies by their im-
pulfe. and to which chey are in pro-
porción (iíec Newto Vrincip, Bbok I
2 .,“— 8.) Buc chis only inte Mo-
tion cannoc be found -out or deter-
inined by us, ñor canwediftinguiih,
when Éwo Bodies any way ftrike
againft each ocher, which the truc
Motion, and confequently the true
Forcé from whence that Im^líe
nrifes, belongs co ; whether to that
which feems to us co move fwifceíií
or to thac v;hich moves ílowefl, or
perhaps feems to be quite at reñs
tiecaufe it cannoc be demonftratéd
whether che Center of Gravity, as
was íaid before, or of rhe whole
Syfiem (which we may properly e-
nough define to be, One Pointin In-
finite Spate,') beac reíforno.

Secondly, Motion rtlaúvely common
is the Change of Sifuation which is
9K/¡de ¿vith rcJptÓi} not to thofe ‘Bodies
whithare nenref^ bnt to fome that are
at adiframe. Andthisfortof Morion
we mean, when weíay, thac Men^
and Trees, and the Globe of the
Eanh it fe!f revoive abone che Sun:
And we mean chis Motion alfo,

when we confider the Quantity of
Motion, or tbe Forcé of a Body in
Motion co ftrike againft any Thing.
ForExample, whenaBallof Wood,
with a piece of Lead in it to make
ic heavy, isthrownoutof our Hand^
^ commonly reckon the Quantity
w Motion, or ibe Forcé with which
the Ball ftrikes, from the Celeruy of
the Baila and the Weighc of the in .
cluded Lead cogecher, I fay vtecom .

reckon i c iba and indeed cruíy,
with refoeéf to the Forcé it íéif, or
any feníible EfFedt ot ic j but whe-
cher thac Forcé or true Motion be
really in the Ball thac ftrikes, or in
che Earth which feems to be ftruck ;
chis, as was laid before, we cannoc
certainly determine.

Laftly, Motion relatively proper, is
the fnccefjive Application of a Body to
the dijfereut Parts of Bodies which
inmeáiately toj^ it» And chis is
the Motion generaliy mean in
Philofophical Difpuces, where wecn-
quire into the Nature ot particular
Things, as when we fay, thar Heac
or Sound, or Liquidnefs, confift in
Motion. But particular Noticeoughc
to be taken, thac the fuccejjivc Appli-
cation of a Body, is fo to be under-
ftood, thac it is to be applied fuc-
celfively to ihedift'erenc Parts of ché
Büdiesimmediacely.touching it, with
its whole Superficies taken together
tout ce tpdil a d'exterienr, as
ih\..^..'firemh exprefíés it j) as when a
Bal]\ is thrown, giides againíl
the diíí5.lhnc Parts of the Air with
its whole Superficies and when our
Handis moved up anddown, ic is
fucGeífively applied its who].e
Superficies, co the difrerhAc-Parts of
the Air on the one Side, the

Jüinc by which ic is faftned cov-he
Body on the ocher Side. It was^'
no purpoíe therefore for Mr. Le Clcr7^JM^
hnd faulc with chis Definición,
K;Vi^ Phyfi lib, y. Chap. y. It will
íays be, thát the Banks and
the Cbannel of the Ri'ver are as
mnch moved as the Water, becattfe
thcy are as fiar removed from the
Water

Chap.io. of Natural Philosophy,

% nothing real in the Subjeót under Coníideration. Thus,
the Man whom we fuppofe walking amongft the Trees,
may alfo keep at the fame diftance from the fatne Parts
of the Water that runs in a Canal juft by, and yet wedon’t
íáy that he is at refl^ and another Perfon fitting in the
Walkj tnay be againft different Parts of the Water, and
yet we don’c fay that he is in Motion. Whence it follows,
that they are very mnch miftaken, who, in order to de-
termine whether a Body be at Reft^ or in Motion, com-
pite it with immoveable Parts which thcy imagine to be

beyond

Water that runs as the Water
is from the other Parts of the Chan-
nel and ’Banks» Biit the Cafe of the
Water ís very different from that of
the Banks, The whole Superficies
of the Water is fucceffively applied
to diíferenc Partsof the Bodies which
0 furround it, and immediately touch
it, and therefore is tronsferred from
fome of thofe furrounding Bodies to
ochers. Buc rhe Banks are parcly fix-
ed to the Earth, and therefore are noc
transferred from thofe Bodies which
immediately íurround them. For
when we fayj that a Body is trans-
ferred, we mean that the Whole of
it is transferred. ^herefore an I-
íland ílickingup middle of a

River, is ñor moved (not ib much as
■with rhis mere relative Motion) tho’
the^Water ílides by it, becaufe it is
firmly fixed in the Earth, and is noc
transferred from thac w'hich imme-
diately couches it. So a Body e-
qually poifed in a Liquor whofe
Parts run opon it with equal Forcé,
is not moved j becaufe though every
particular Pare of the Superficies of
it be every Momenc applied to dif-
ferenc Parts of the Liquid thac íí^
rounds ir, y'ec che whole Supér^^R
of it is not transferred at on^roin
the concave Superficies of^e Parts
which íurround it, confidered as one
whole Superficies^,

Further, a^É^ng to chefe dxíFe-
. íenc E)e&¿i(mis of Motioriy are we to
undeiJfBr^che Word Place in dif-
ferjy Senfes.. ' Por when we fpeak
or ahfolntely -proper Motion
BPJor Repi) then by Place we mean,
that Parí of infinite and tnimoveabl^
Space rohich thc ’Bodypoffcffes j wh||fl
we ípeak of Motion rclativtly(^f^
wony then by Place is meant, a Part
of fome particiíliir ¿'pace or move-
ableDimenJíorji which Place 5c fcifis

truly and properly moved, along with
that which is placed in it : And when
we fpeak of Motion relathely proper
(which indeed is very improper)
then by Place, is meant ¿¿e Superficies
of t^l^od'ies {ox fenfible Spaces) xohich
imrnediatcly furround the tbing moved*
As to che Dtífinifion of Peji, all
are very weil agreed in ic : Buc whe-
ther Reft be a mere privation of Mo-
ción, or any Thing pofitive, chis is
fiiarply difputed. Caries and fome
others concend, that Thac which is
at Reñ, has fome kind of Force> by
which ic continúes at Reíf, and
whereby ic renfts every Thing thac
would change its State ,* and thac
Motion may as well be called 3
Cefíácion of Refl, as Reft is a Ceílá-
cion of Motion. Malebranch in his
Encjtiiry after Trnth, üook 6» Chap, 9.
and others contend on the contrary,
thac Reíf is a mere privation of
Motion ,• their Argumeots may be
íeen briefíy explained in Mr, Le
Clero' s Phyf. 'Book y. Chap. y. One
Thing only I %vould obferve by rhe
way, relacing to thisMaccer, and thac
is, rhac Malebranch and Mr, Le Clerc,
who follow his Opinión, in the
following Arguraenc, beg the Que-
ñion. Suppole, fay they, a Ball at
reñ: ; fuppofe that God íhouJd ceafe
to \vf!l any Thing concerning it;
whac T/ould be che Confequence ?
Ic would be at reft fiill. Suppofe it
be in Motion ; and that God Ihould
ceaíe to Will that ic íliould be in
Motion, whac would foFlow then ?
It would noc be in Motion any lon-
ger. Why noc? Becaufe che Forcé,
whereby che Body in Motion concinu-
ed in the State i c was,is x\^epoJitiveWi\\
of God, but thac w'hereby iris acReíl
n only privativo : This isamanifeñ
beggingof cheQueñion. Inrealicy, the
Fí>n:eor2cn/íí;/f)'bywhichBodieSpWhe-
ihef

4E

4í

4.^ Yemári-
able Jnjiance
cj a ’Boáy in
Motionand of
another 'Bady
at Keji,

S.'That to r¿-
fíl fome fort
fif Motion^ Í5
to move to-
so ardí the
íontTary^art»

6- That Mo-
t:on and Reji
are only
Modos ef ex-
and
Axt each of
íheyn but
^ccidents of
Masíer,

ROHAUL^'s System PaM/,

beyond the Heavens, where it is very uncsítainj whether
there be any Parts of Matter more immoveable than thofe 1
near us. !

4. Having thiis explained the Nature of Motion and ^
Reftj when we fee a Fiíh in the River keeping it felf
for fome time right againft the fame Part of the Bank,
and neither the Stresm which furrounds it^ carrying it
downwardj ñor its own Forcé, by which it ftrives againft j
the Stream, cafryinglt upward, we fay that it is really in
Motion, becaufe it really agrees in every particular, wich
another in a Pond, which is by all allowed to be in Mo-
tion j for the Effort of the Former, makes it to be fuc-
ceffively applied to the diíFerent Parts of the Running
Stream, in the fame manner, as the EfFort of the Latter,
makes it to be applied to different Parts of the Water in
the Pond. On the cc^rary, when we fee a Stake float-
ingon the Water, and rarried alongwith theStream,we fay
that it is at Refi, becauíé it is incompaffed with the fameJ
Parts (which is the general Reafon why we fay a Body is
at Reft) thoughat the fame time, the Stake and the Ri-
ver together, are but one Thing in Motion.

5. When a Fifti that moves it felf in the manner
now defcribed, is not carried along with the Stream, we
are ufed to fay, that it refifts the Stream ; fo when a Bo-
dy by its Refiftance, hinders it felf f?í»ñ being carried
along with another Body with which it is entirely fur-
rounded, we may as well fay, that it moves the contráry
way.

6 . Becaufe we cannot conceive any Application to
different Parts, without fuppofing a Body fo applied, fo
that Motion depends necefíárily upon the Thing moved ;
therefore we are not to think that Motion is any real Being,
but only a Moíie of the Body in Motion; and fo likewife,

■ that Reft is only a Mode óiw^e Body which is at Reft.

Whence it follows, that Moiií^ and Refl add nothing
more to the Body in Motion or ai^níf/?, than Vigure does
to a figureá Body ; and fince a Body may .either be moved ■

pOFííon to theír Deníir^d*-<hqt ísi to
the (^aniky of Matter coñíS c.ed in
them ; and every Body flrikin^eip-
on another with a ^iven
whether that oiher be greater oW/ll
ie/s, moves it in proportion lo che
or Quantity of

tf^ ií)ne, to theDenfity or Quantity
of Matter in the other.

ther in Motion or at Rejii continué
in the State in whicti they once are j
5s the mere Inercia of Matter j and
therefore if it could bes that God
íhould forbearwilling acalla a Body
that is once in Mocionj would move
on for ever, as 'welI as a Body at
continué at Reíf for ever. And
the EíFeíf of chis Inercia of Matter
h chis; that all Boáiesrehíl in

or

Chap. ip. c/ N atural Philosophy. 43

or not moved; we conclude, tliat Motion and Refl: are
pnly accidental to Macter.

7. Motion has always been acknowledged to be a Spe-
cies of Quantíty, which is meafured partly by the Length
of the Line, which thp Body in Motion runs ; fot Ex- Motknf
ampie, when a Body of a given Bignefs, fuppofe a Cubic
Foat, moy.es a given Space, fuppofe fixty Foot, we cali this
a determínate Quantity of Motion, and it is twice or thrice
as much, if the fame Body runs 120 or 180 Fget.

I. It is alfo partly meafured * by the Quantity of Matter ^^«‘ther
which moves together ; For Example, If a Body of two pretil
Cubic Feet runs through a Line fixty Footlmg, it has twice évmútyof
as much Motion, as a Body of one Cubic Foot, which runs
through the fame Line ; For it is evident, that we ought
to reckon as much Motion, in each half of the Body of
two Feet, as in the whole Body of one Foot.

9. Whence it follows manifeffly ; that in orderfor une- 9 - 'w»

•qual Bodies to have equal Quantitics of Motion, the Lines ’dÚTmly^avc

which they run through, ought to be in reciprocal Pro- e^tti
portion to their Bulk. Thus, if one Body be three times
as big as the other, the Line which it runs through, ought
to be but a third Part of that of the other.

10. When two Bodies hung at the Ends of a Balance lo.How tu¡>
or Leaver, ar^o one another, in reciprocal Proportion

to their Diftan^W from the fixed Point,- they muft ne- ofa ■Baiiance
ceflarily, when they are moved, defcribe Lines which are f"
to each other, in reciproca! Proportion to their Bulles. ‘

For Example : if the Body A be three times as big as the Tab.i.Fig.j.
Body B, and thefe Bodies be.fo faílned to the Ends of the
Leaver AB, whofe Point C is fixed, that the Diílance BC
be three times as much as the Diftance ACj the Leaver
cannot incline either to the one Side or the Other, but

the Space BE along which thñleller Body is moved, will
be three times as much^^the Space ÁD along which
the greater Body is mQJp!^ wheretore the Motion of the

I . Sy tht §ÍKat}tí^^/^tter) That
is, of the MaítflgpplT^ belongs pro-
perly to ih^Boy i n Motion ¿ For,
che lubjl^TOatter, if there be any
fuch^F^g» wich which the fmall
Tof terreñrial Bodies are filled, is
ITc cransferred along with them,
"wich the fame common Motion 5
Therefore if a BajI of Irona andi|l
Ball of'^ood of theTame BigapP
be moved with the fame Celei^a
chele will be more Motion in the
Ball of Iron, than in thatofWood.
Sq likewiíé, U iwo eijual leaden

Balls^ the onefoHd, the other hoIloW
and empey, be moved with che fame
Celerity ; che íoüd Ball will have
more Motion than che hollow One,
and wili ftrike a Body againíl which
je is chfown with greater Forcé. And
the Quantity of Macter which is pro*
perly contained in any Body is to be
determined by ics Weighe. Where*
fore tha C^ianticy of Motion is not
to be meafured by che Celency and
Bigneís, but by che Celerity antj
VVeight of the Body in Motion,
which is cafefuUy to be obíérved-

Qiie

44

li.TheKea-
fon vohy Li~
^Hors ballance
each other.

Tab.I.Flg.4.

ROHJULT^s SvsTEM Partí.

one Body, will be exadly equal to the Motion of the
Other. This being ío, there is no Reafon to thinkj that
the Body A, with four Degreesj fuppofcj of Motion
downwards, íbould lift up the Body B with four Degrees
of Motion, rather than the Body B with four Degrees of
Motion tending downwards alfo, íhould lift up the Body
A with four Degrees of Motion j wherefore we ought to
think that they will be in aquiübrió. ' And this ’is the
Foundation of Mechanicks.

1 1. So likewife when any heavy Liquor is contained in
an inverted Siphon, whofe Tubes are wider one than the
other j if we imagine the Height of the Liquor in each
Tube to be divided into a great many equally thin Planes,
one of thefe Planes in either Tube, cannot by íinking,
raife the Liquor in the other T ube, but the Sinking and
the RiQng muft be in reciprocal Proportion of the Quan-
tity of Parts which íink to thofe which rife. Thus, if
the Width of the Part AB, the larger Tube of the Si-»))
phon ABCDj be a hundred times as much as the Width
of the Part C, the ftraiter Tube) and confequently,
the Quantity of the Parts of the Liquor in the Plañe A B,
a hundred times as many as the Quantity of Parts in the
Plañe C ; then the Riíing or Sinking of the Parts on the
Side A B, will be to the Riíing and Sinking of the Parts on
the Side C, in a centuple reciprocal Pr^Jíortion ; Where-
fore the Motion of all the Parts in the Tubfe AB is ex-
actly equal to the Motion of all the Parts of the Tube C.
So that they in the one, are no more ableby fmking, to raife
thofe in the other, than thefe Latter are able by íinking
to raife the Former. Whence it follows, that if each
Tube be divided into an equal Nurober of Planes, that
is, if the Liquor be of an equal Height in rhem both,

I it muíl keep it felf in ¿ei^mlibrio, unlels diílurbed by
fome external Cauíé. 12. Since

Tab. I.

í'is- ■

I. ^nd this is tht Fonndation of
Mechaniibs) Upim rhis ís built thác
famous Problem oi Archi-
mides-, A05 ttS íw ^ 'tÍw
zivi¡ir^, 2o move a
given Weight xoich a gi~
'ven Forte'. For by increafing the Dí-
ilance CB, the Forcé of theBodyB
may be increafed infinicely. For the
manner how this is done by increa-
íing che Number of Leavers, Wheks^
Píilleys, Screws, fíe. Sée FFilbins’s
idathematical Magicb-, and ochers.
The Forcé of every one of which
Jyicíhjmick Powgrs, and whence it aii-

fesrS^i'i^Iy explained below in the
Notes o".*.».:!!.!:' i4fA Chap. Art^c. 5-
I . It mñjl it felfin equilibrio.}

Henee it folIlí5^ Thnt aíl Li-^
qiiarsprefs upan SortXÍ
that are nnder

according to íheir per- FigPvAjj
pendicttlar Height jünd a'"' ij

noi according to their ‘Breadth. Wf?í^ J|1
Paradox may alfo be demonflraced in |
, ^hefóllowlpg Manner. Lee ABCDFE
^t^a Vefíel filíed with Water; Now
Decaufe che Culnmn BF is heavier.
chan the Column HGj it ismanifeíij
that if the Vefl'd were open at H»
ihe Column G H would rife cill ic

becamq

Chap. 10, of Natüral Philosophy,

12. Sílice it is only che EíTencial Properties of any Sub-
jeü, which can be deduced from che Effence of itj afcer
ic is known,' ic is to no Purpofe for us to endeavour to
find ouc how Motion could be firft produced in Bodies,
becaufe chis is noc an eílential Propercy j we íliall not there-
fore ftand co argüe upon chis Subjedi: Buc as we own
God to be che Creator of Matterj fo likewife we own
him to be the firít Mover of iC.

became in aquilibrio with rhe Co-
liimn B F. Since therefore the Co-
ver which íhuts up theVeííél at H,
hinders che Column GH from riíing,
ic ís evidentjChaccheWacer ar H prellés
the Cover of the Veflel upwards wich
a Forcé equal co the Weighc of BL j
and becaufe all Preflüre is reciproca^
ic is evidenc alfo, chac che Wacer ac
G preíiés che Botcom of che Veflel
downwards wich che íame Forcé j
co which Forcé cheV/eighc óf che
^Column G H is to be added> by which
means» the Forcé of che Water pref-
fing lipón G> will be che fame as if
the Column GH were equal in heighc
to che Column FB> chac is> as ii ic
were filled up to M. The fame may
be demoníiraced likewife of all che
ocher Columns j whence ic is mani-
feft, chac che Boccom E D is preíTed
in che fame manníl||^if che Veflel,
every where of equa^rwickneísjwere
filled wich Water co N O,

Buc che Truth of chis Demonftra-
tion depends upon chis Suppoflcion,
Chat che Liquor concained in che Vef-
fel be fuch as cannot be compreíled :
as Water which cannot be comprefléd.
Whac therefore was faid of all Li-
qrtors^ is to be underílood of fuch Li~
quorSiVil. chac chey prefs ttpon ‘Bodies
thatare tmder them, accordhtg to tbeir
perpendicular Height» and not ae-^
cording to their Breadth.

CoYoU I. If cheTube AB be
ped clofe wich a Cover, and^jWic-
cle TubeCDbe ^^prwich
Tab.I, Wacer up Wacer

Fig. 4 . concain^j^msTube, will
preíaM^che Wacer below
in che aÍ||PÍube, and chis Preflüre
will JjBue ic felf through all the
, and chruñ againíl che Sides

L|g[ÉrGover of che Veflel ihusciofedj
P^md if a Mole be made ín che Cover,
for che Water co gec ouc ac, ic wifl j
By quc 'chence wich as much
as if the iictle Tube CD wer¿-^2?^
broad as che Tube A fi.

CoroU i. IftwoCylindersbe exa£i-
ly ficced co che Tubes AB,'

C Da Weighcs laid upon Tab.I.
chem will be in aquitibrioy Fig. 4»
if chey are in proportion co
che Width of the Tubes. For Ex-
ample, if the Tube AB be four times
as wide as che Tube CD, one pound
Weighc laid upon che iiule Cylinder,
will be equal co che Forcé of four
Pound Weight laid upon the greac
Cylinder s which Experimencs may
be infinicely diverfifyed.

Corol, 3. Henee ic is eafy co ex-
plain chac Paradox, which ib much
perplexed the Famous
Dr. Henry Mootí and Aab.XVII.
other learned Men, •vicc. Fig. 2.
why a flac round Board,
fuch as a Trencher, when ic is puc
inco Wacer, íhould rife up immedi-
acely, though the Weighc of che in-
cumbenc Wacer be much greater, than
chac under ic, and yec ebere be no
fuch Thing in.Nature as Lightnefs
co lifc it up. Lee ABCD be a Vef*
íél full of Water, F a . round Board
immeríed in che Warer. Now be-
cauíe, from whac has been already
íáíd, the Columns of Water Ub, Ub,
communicace.all cheir Weighc co che
Column ády and, if the Column dd
íhould defeend, che Column
^ould alcend with a Celerity, pro-
^orúonably greacer, . as chey are lefs
cíiick j whence ic is evidenc, chac
thefe oughc co be in aquilibrio wich
each other (in che fame mannér as
in che SiphoD, 7 ví¿. 1 . F%.4.) if che
Column d'á be all Water. Buc be-
caufe pare of chis Column is not
Water, buc che Board F, which is
fpecifically leís heavv chan Water ■,
therefore che icqulUbrium is alcered,
and che Column GGdd baving leís
Forcé (compounded of che ívíagni-
cude and Veloclcy) chan the Columns
■Hb, Hb'i ic.rqufl: rife fo far, chac
there muíl be as much of the Wood
'abüve

45

i^'^hatGod
is the jirji
Moviti

4 ^

That it
is fuffíátnt to
aUoroi that
God once ere-'
aeedMoíiott* .

R O HA U L T^s System Part L

13. But becaufe it is not thc Part of a Philofopher to
make him working Miracles every Momentj and to have
perpetual Recourfe to his Power, we fhall take it for
.granted, that when he created the Matter of this World,
he impreíled a cercain Quantity of Motion upon the Parts
of it, and that afterwards, by the common Courfe of his
Providence, he hindred Things from returning into their
original Nothing, and preíerved always * the fame Quan^
tity of Motion í fo that what remains for us to do, is on-
ly to enquire into other Circumftances of Motion, and
to examine Sccond or Natural Caufes.

above the Superficies of tbe Water,
as it exceeds in Bigneís a Quancicy
of Water of equal Weighc If che
round Trencher F were fo exaftiy
ficced totbe Widch of cheVeflél, chac
no Water could gec between it and
the Sides of the Vefiel, íb as to com-
municate its Weighc to the Water
below, and by chac meaos forcé che
Board iipwards ; or if che Board
touched the Bortom of the Veflei fo
clofe, that no Water could gec in
between it and che Boctom, then
the Board would not rife at all. As
I have often tried in Quickíilver,
■which does not wec the Board, and
therefore will eaíily let h go cloíe to
the Boctom of the Veflei.

I. The fame ^tanúty of Motion)
Some other Principie (befide the Iner-
tia of Matter) veas neceffary for put-
img Jodies into Motion j and novo they
are in Motion^ fome other Principie if
neceffary for conferving the Motion .
Forif tvoo Globes joined by a fender
Rod^ revolve ahont their common Cen-
ter of Gravíty roith an uniform JVío-
tiony vohile that Center moves on »ni~
formly in a right Lhie draxon in the
Plañe of the circular Motion j Tíf^
Siim of the Motions ofthe tvoo Globesf']
as often as the Globes are in tbe
right Lint deferibed by their common
Center of Gravify, roill be bigger than
the Snm of their Motions, vohen they
are in a -Linc perpendicular to that
right Line, *By this Inflame it ap-
pears, that Motion may begot or loji. 3 y
reafon of the Tenacity of Fluids, and
Attrition of their Parts^ and tht
Weaknefs of Elaficity in Sollds, Mo-
tion is mtich more apt to be loji than
goty and is always upon thc DccayÁ
For Hodies vohich are eitker abfoluíef
¡y hard, or fo fojt, as to be void of
Flaficityy tetll not reboimd from one
anoíher, Impenetrability makes them

only Jlop* íf tvoo éqttal *BodÍes meei
direhly in Vacuos they xoill by the
Laxos of Motion fiop vohere they meety
and lofe all their Motion, and remáiii
in Rejly untefs they be elajiieb, and
receive nevo Motion from their Spring»

2 f they have fo much Elajlicity as ftif-
fices to make them iebound, voíth
ijuarter^ or half or three qitarters of'^
the Forcé voith which they come toge-
ther, they will lofe threc ¿luartcrs, or
Half, or a ^Inarter of their Motion.
^nd this may be tried, bylettingtwo
equal Pendulims fall againji one anoj
therfrom equal Heights. If the Peri-
dnlum - be of Lead ot foft Clay, they
will lofe all^ or almo.fi all their Motions í
If of elafick^^rAgs, they will lofe all
but what they recover from their Ela^
fiieity, Newton*s Opticks che ad
Edición, in Englijh, p 373»

If it be asked how Motion. which
is ihusperpecually lofl:» fliould be per-
petually regained. The Anfwer is;
That it is regained by certaln aífive
Principlesj fuch as are the Caufe of
Gravicy, by which Planets and Com-
meis kcep their Motions in their Orbs^
and ’Bodies aequire great Motion in
falUng. The Caufe of Fermentación ,
which the Heart and ‘Blood of
ll/iLfyals are kept in perpetual Mo~
tion iil^li^Heat : the invpard Parts of
the conjlantly warmed, and

in fome Pli(c\íc.)CTovo very hot, 3 o-
dies burn and)t.f 7 %^ - Motintains take
Fire, the CavernsffJ Earth are
bhwn Hp ; and che Sttn^íy'^yfs vio*
lently hot and lucid, and w^^ ’f all
Things by his Light', (and rhe'í^^ i
of Elafticíty whereby Boqíss reífe?^ MÍ
themíelves to their former Figures; I
'•flll which Caufes fhall be created of
ijheir proper Places) For we meet
wkh very little Motion in the World
befdes what is awing to thefe añive
Principies. Ibid. p. 37^.

Ch A F,

Ghap. lí. o/ Na tu RAL Philosóphy.

47

C H A P. XL

0/ the Continuation and Cejfation of Motion.

TTO W it comes to país chat a Body in Motion, fhould i. That ¡c
continué to be moved, is one of the moft coníi- "nndyatHep,
derable Queftions relating to Motion, and has yery much %" feif^"e¿n
perplexed che Skill of Phílofophers ; but upon our Prin- to mevCy ñor
ciples, ic is not difficult to account for it : For, as was Óf it
befóte obferved, nothing tends to the Deftrudtion of it /«// d
felf, and it is one of the Laws of Natnre, that all Things “««•

'luí// continué in the State they 07 ice are unlels any external
Caufe interpoíésj thus that which exifts to Day, will
cndeavour, as far as it can, to exift always j and on the
contrary, that which has no Exiílence, will endeavour, if
1^ may lo ípeak, nevef to exift ^ for it never will exift of
it íelf, if it be not produced by fome external Caufe : So
alfo, that which is now a Square, will, as far as is in its
Power, always continué a Square. And as that which
is at Rett, will never of ic felf begin to move, unlefs
fomething iiMve it; fo that which is once in Motion, will
never of its reft ceafe to move, unlels icmeetswith íbme-
thing that ret^|^or ftops its Motion. And this is the
true Reafon why a Scone continúes to move after it is
out of the Hand of him that throws it,

2. We íhall therefore have but little regard to that com- rhat u «
monSaying of ^r/}?oí/e’s, That everyThing hi Motion tends “Tmjiakc to
to Refl, becaufe there is no good Reafon for ic. For if

this Opinión feems to have fome Foundation from what tion do of
we experience on che one Hand of the Things on the „

Earth, where a Stone or any other Body in Motion does
not continué always to yet ic is overthrown by

what is obferved on cl^Other Hand in the Heavens,
where from the ObáÉwtion of many thoufand Years,
we find no Dinikjíllon of Motion.

3. To wh^jpi^e may add, that this Opinión is not fo ,, ^ri-

eafily by the Experience of what is done here üotie', opL ^

upoB^me Earth, as is imagined : For though indeed ic í

L^drery evident, that we fee the Bodies which were in Mo- ¿xpkLt/
^Kon, ceafe to move, and to be ac perfed Reñ ; yet it is by
po means evident, that the-Atend to this of themfelves :

For no Body can ever th>^s that a Cannon-Ball, after ic
has encered three or four Foot into a Wall, has an In-
clinación after that to be at Reft. On che contrary, when

we

48

RO H AU L'T’ s System Part I.

wé perceive thar this Ball enters deeper or lels deep, ac-
cording co the DifFerence of the Bódies that receive rhe
Forcé of ir, we afcribe, with more Reafon, the Cella-
tion of ics Motion co the greater or lefs Refiílance made
by thofe Bodies.

^.Thatíhe 4. This Opinioti was peculiar to and 110 Body

"íZtZ^md. would have evcr come inCo it, if they had confidered,
that the Re- that Air, though ic does noc relift Motion fo much as a
^eTií°tiíe°' i^^^kes fome Refiílance, as we experience in

Caiife efe- ^ Fa'i moved quick j for then when they had leen a Can-
ther 'Bodies non-Ball or a Bcone, noc always continuing to move in
OTTOí? '■^^y would have thought, that chis was caufed by

the Refiftance which the Air makes to che Motion of the
Ball, and that che Ball lofes as much Motion as it commu-
nicates to the Air.

j-. That a 5. .Now in ordcr to find out how much of its Motion
Body in Mo- g Body lofes when ic ílrikes againíl other Bodies, you muft
mmhof%s^° remember, that we fuppoíed i that God created a certainj^
oan motion Quantity of Motion, and that by the common Courfe ot
Zicates’"to‘‘ó- Providence, he preferves as much Motion in Mac-
ther 'Bodies. tcr, as he imprcfled upon ic at the Beginning j whence
it follows, that if a Body in Motion, ñrikes diredüy up-
on another Body at Reíl, and pulhes it befor,e it, ic muft
neceílarily lofe as much of its own Motion, av.ic communi-
cates to the other, in ordcr for them to together with ,
che fame Celerity as if the two Bodies were one common
Maís. Wherefore if a Body in Motion be three times
as big as the Body at Reft, ic will lofe a fourch Pare of
ics Motion,- and inftead of running, fuppofe, a Line of
four Fathom, in a given time, ic will run buc a Line of
three Fathom; that is, ic will move with a fourth Pare
lels Celerity, than ic did belore.

6 That a 6. If a Body in Motion, ílrikes upon another Body in
Body in mo- Motion alfo, it will makeid¿-’r move í'wifter; buc ic will
oPiumítim, much of its owmR'tJ^ion, as if this latter had

Khenitftribe's been wholly at Reft; becaufe ^;^'*ac it hasco do, isonly
"tí^nBoT’ñi- ft>me Degrees of Motion tolvi'^-.^e^^it has already, in
\cZáy i/mo- ot^et CO make the Bodies move v/ith SL Arpe Celerity ;
tion, than One Example will make this clear. SuppSit.^., Body to
^‘n“/Body ^ certain Quantity of Motion, for inílanc^^velvc

«í Refi, _ L

I. That God created a certain | noc refleiíled» biic lofe tbeir MotiSiJ I|r
§^antity of Motim) See above | yec in other Cafes, Bodies perfedlly j
X. Art. Buc though Motion^hard, communicate theír Motion to
may be deílroyed? and hard Bodíes pC^iih otñerj according co chufe Laws
that have no elaftick Furce, v/hen I wíucb che Author is- explaining.
they. ftrike againíl each ocher, are I

Degrees,

49

Chap. II. of Nátiíral P hilo^oph y.

Degrees. and that it ílrikes upon another which is aC
Reíl; according to what was now faid, if tlie firft Body
be as big again as the otheri it oughc to cOmmUflicate
four Degrees of Motion to it, and keep eight to it felf.
But if the Body which has twelve Degrees of Motión, ílrikes
againíl the other tnoving with three Degrees, it ought to
inereafe its Motion but two Degrees, to make it have as
much as it ought to have ; becaufe this being but half as
big as the other, it will by this means have Motion e-
nough to go as fwift as the other : And therefore that Bo-
dy which before kept to it felf oniy eight Degrees of Mo-
tion, will now keep Ten, *

1 . if a Body in Motion^ be three
times as big as another Body ac Reft,
and ftrilces againílic with thirty two
Degrees of Mocion¿ it will give it
eight Degrees of its Motion> and
^ Iceep Twenty four to itfelf; But
if the latter Body had four Degrees
of Motion before, ic will give it but
five Degrees, and keep twenty Se-
veo. Bythefame wayofReafon-
ing*, it is eafy to find out other Laws
of comtriunicating Moción in Bodies
that are perfei^ hard. But becaufe
íhe hardefi: all have alio an

W.lafiicl} ForcCy ai^^aufe the Caíe
of Elaílick Bodiesp^Rüfferenc from
this, and more difficultj you mayfind
the Principal Laws by which their
Motion is commünicated, explained
by cheíeleafned Períbns; Sir fAn-
Ji'ophcr VVren, Dr. Wallts^ Mr. Hu~
¿íwíj in his Philofophical Traníadli-
oñs, Numb. 43, and 46) and more
fully by the fame Mr lingtns in his
Poílhumous Works^and by Mr. h'iar-
ñoti in a whole Book wroté upon
this Subje61:3- and alfo veíy tully b;
Dv. l{eil in his Leólures upon
tiital Philoíophy. But
Matcer may be conáprehei^
following

^nts and Velocicies with
fwo Spherical Bpdíes,peffe( 5 l-
Hck, whofe .Centers are mo-
í in the fáme ílreight Line, .meet
) othera being gíven j to find their
Velocities aftef tbey have met. ““
in the following Computacionr-
i^-lotion of Elañick Bodies after'^i'ííi
king againíl each other, is fuppofed
aoarifefrom twó Caiifes»

I. From fimple Impulíé. By the
Forcé of which alone, if the Bodies-
had no Elafíick Forcé, each Bo'dy af-
cer cheyhad mee, wouldelther whol-
ly reíl, vix., if they meet each other
with equál Motion,' or they would
go both on Eogetner, as if they were
United into one Body j ■wichthefame
Velocity ; and the Sum of their Mo-
tions (if chej* moved both the fame .
Way) or the Difíerence of their Mo*
tions (if they moved concrary Ways)
would continué the íame afeer their
meeting as before.

II. from ElaJiick'Fürce. Which
in Bodies perfe£lly Elaílick, isequal
to the Forcé with which they are
compreíTcd i thacis, when twofuch
Bodies afe ííruck againíl each other,
ic ¡s equivalenc to that Motion which
eichef of efiem would gai ñor loíe by
fimple Impulíéonly: This Forcé a¿ls
che concrary way, and therefore the
Mocioh which is pfoduced by icj
raufl be fubílraíled from that Mo-
ción, which isin theBody impelling,
Éndadded co that Motion which is
Tn the. Body impelled by the Forcé
of fimple Impulfe^only, in order
to find their Velocities afeer Refie-
clion¿

This being íúppoíed. Lee A and
B be two perfealy Elaílick Bod/es,
and lee A either overeake B, or meet
if, Lee their Velocicies be a and
¿f ¡ ThencheMocionof A wilIbeAtfj
and the Motion ofB, wlUbeBí», and
chéQuantity of Motion, in chem
both cogecher, if they be moved the
fame. or concrary ways will be
Aa -¿Bb^whichlby the tít Po/tc/'o»)
will be che fame afeer their Impuife
as before. Now (if theyhadnoE*?
laftick Forcé) their common Velo-
E City

5^

7 - Hoto ■
^Body lofes ií
Motion»

RO H J U LT’s Sysi'em

Part L

7. If a Bod7 which was moved by another, be by any
Means tumed out of the Way^ fo thac Thac from which
ic received its Motion^ is left to move freely, ic will con-r
tinue only to move as it did after it had moved the otherj

and

city after they had met, would be
Aa ir Bb ,

, and therefore the Motion

•B

of Aj would be
ABa±

A^aír ABb

Chat of B,

A +
B^b

and

• B

Now if the

Aaaír ABb , . .

Motion — . — > which remams

A +- B

in A after the Impulfe, be íubñract-
ed from the Motion Aa, which ic had

at firñ, chere will remdin che Motion
ABaor. ABb ,

which the Body A has

A ■• 5 “ Já

Jofi: by Simple Impulfe only. Nov/' if
chis Motion be fubílraíírcd from the
A^a^ AKb

Motion

• B

which is in A,

and added to the Motion

ABa:í=-Bzb

¿I *4“ B

which is in B after their Meeting, from
che firít Caufe oniy ¿ the Remainder
Abatir 2ABb— ABa ,,

wiJJ (by che

A •+“ B

lecond rojition) be the Motion ot

A -4— B

will be che Motion of B, from borh
Caufes togecher, afcer ReBedion.
And by díviding íeparacely theíé Mo-
cions by thcír Bodies. we íliall have

AadtzBb- Ba . , vr 1 • r

for the Velocicy or

A .

A, and

2Aa.ir Bb^ Ab ,

Meeting, either reft; or go on to be
moved the concrary way to that A
was moved in, before they mee 5
and then the Quancicy by which che
Velocicy is expreíl'ed, will either be
Nothing, or (as ac firíl) >jegacive.
Buc if ic bé driven back the íame
way thac A was moved in at firíl,
che Quancicy by which the Velocity
is exprefled, will be pofuive. For
fince the Velocicy that way which A
.was ac firíl moved in, is exprefled
'by the Sign j 'tis evidenc, thac che
Velocity che concrary way, oughc to
be exprefled by the concrary Sign
— chroiighouc the whole Computa-
ción.

Tromebefe general Quandciesnow
fnund, by which the Velocities of
che Bodies A and B are exprefled, ic
is eafy to deduce the Laws óf Moti- •
on wlilch are obferved by any per-
fedly Elatlick Bodies ^cer Reflexi-
ón, in any given C'd^ whatfoever.
For Example,

1. If the Vg^t^*i.ésof two Bodies
meeting each otner, be reciprocally
as their Weights, in chis' Cafe ic
will be Aa ^ Bb, and therefore the
Quancicy by which che Velocity of A

n*j —Aa— Ba
is exprefled, =;

and thac of B, ^

A •\
Ab •

•B

- Rb

, ,, -for che Ve-

A -t— b s

locicy of B after KefleSlion. Q^. E.

(See Necot. yHgehra, 91*

I*rohL 12.)

iV. H, It may ío happen, that the
Body A, whether it ovcrcakes B, or
nieecs ic, may lofe iill íts Motion, or
may be drivenback che contraiy way
to that ic moved in before ihey mee.
Wherefore in chis Guíe clie'Quanciiy

Aa**“2Bb-- Ba , u- u u -¡7
— — by which the Ve-

locity after RefleXion is exprcíléd,
will efcher become Nocbíng (die Nc-
gacive and Pofiiive Terms deílróying
one another) or Negacive. So like-
wife it may happen, thac w'hen the
Body B msecs Aj ic may, ^fter their

A -f— b

Thac is, each Body after their Im-
pulfe, .will go back wich che fama
^ Velocicy wUh which they mee each

pjf A ílrikes agalnír B, when ic
is ac->^,^ che Velocity of A will ba
(the B, and confequenrly

its MuUipIcíírj.'^- €^ír. vanilhing) 5=

Aa '

IT-

Ba

1

. and coü^.^ii^icy of B will

be

2 -A a

That is,^'^.>'

D)

D. .

of their Bodies is to their DifFerenVvs'v.
fo is the Velocity of the Body A
fore Reflcdbton, to íes Velocity af- ■ ^

S efleXion. And as che Sum of
f;di?s, to düuble the impelling
^ , f) i.s che Velocity of A be-
fore Reflexión, to the Velocicy of
B after RefleXion,

fl. If

Chap- II. of Natural Philosophy. BÍ,

and noc as it moved before ic comníunicated any of its
Motion j becaufe the Mann^r in which any Thing oughc
to continué to exiílj and co preíerve ic felf, is thac whicb
it has chis very Momento and not thac which it had íbme
E 2 Time

3. If A be equal to B, and firikes
agamíl ic when ic is ac Reíi, the Ve-
locity of A will be ^ o. And che
Velocity of B will be rn a. Which
íhows cbattheBody Aafcer ílriking,
will beac Reíl» and the Body B will
be moved wich the fame Celericy
afcer che Impulfe, thac A was mo-
ved wich before cheirapulfe.

4, It A and B be equal> and meec
eachocher wich unequal Velocitya che
Velocity of A afcer meecing will be
£=; — bj and the Velocity of B s a.
Thac is> each of them will return

1^ back afcer meecingj having changed
their Velocity.

y. If A and B be equal , and A over-
lakes B, che Velocity ot A will be
b) and the Velocity of B =3 a.
Thac is, they will both move the
fame way they did before, having
changed iheiAVelocity.

L^^A.

If there be three unequal Quanti-
ties Aj B, C ; and A be leís than B,
and B lefs than C. I íay, (i.) that
AC _

B is lefs than A +-C (2.) that

AC

B - 4 - "r* isleañ of all, when B is a
H

mean proporcional becween A and
C,

B E M O N S T.

•íhe firft parrisevidentfrü
üy. 'BooA’ s,Qf Eíiclid.^^mFSecorid
Pare may be demonf^Urthus. Lee
M bea mean^tfpPffional betv/een
AandCjtíápIRi =; AC. Now
xf M^gf^obe equal, ic is B *f.

aMorzB. Bucif there be
difFerence becween M and B, lee
^thac diíFerencebe D i and ic will br
Mi A

M •

:D +.

M •

.Mi

^ B -I-

panng thel r Produéls cogether.There*
íore, &c. Q. E. D.

i6>) Lee there be three Elaífick
Büdies, as mentioned in che Lemma,
A. B, C i and lee A ífrike againll B
ac reíi j and afcer rhar, lee B ílrike
againfl: C ac reft alfo j I fay, thac by
chis Means, che Body C will aequire
greater Velocicy, than ifit had been
ftruck immediarely by A alone, wich-
ouc rhe Incerpoficion of B 5 and thac
ic chen aequires the greateíl: Velocity,
when B is a mean Proporcional be-
cween A and C, (And the íame
holds crue, if the Mocion begins
wich the Body C.)

For by che SecondL^Wt explained
above, the VeiocicyofC, ificwere
impelled by A only, and-the Body B

noc beiween them, wiUbe-^-^

. A — C

4 A a

or And by the lame

2. A “í — 2 .

Law, the Velocity of C, when ftruck
by che Body B v'ith thac Modoa
which was given ic by A, wilí be
___ 4. A a

Biit M 4- B greater than

a M as is evident by mulciplyir.g
cachof them by M D and cum-

A 4_ c Ht- B 4- A c, which cwo

*17

Fra£lions, becauíe they have the fi me
common Numerator (4 A a) are ro
one anucher as cheir Denominacors,
inveríely. Whereforethe Velocity of
C in che lirft Cafe, is coits Velocity
'n ihe Second, as A B 4 -

A C

— to 2 A 4— a C. Buc (¿y the

^ A C . , ^

Lemma) B +* -g- is lefs than

A 4- C, andleaftof all when A, B,
and C are in concinual Proporción.

Therefore A 4- C -f- B 4-

B

is lefs titán i A -h-2 C. That is, che
Velocity of C, in the firll Cafe, is lefs
than its Velocity in the Second, and
this Inequality is greateíl, when
A, B ande, areincontiniml Propor-
ción. Tf the Motion begins at tlie
Body C, than if c reprefents its Ce-
lerity, and befubrtitutedin theRoom
the Demonílration will be the
fame. 7- 1’1«

$2

8. That
; reater 3 o-

ROH AULt^ s Systém Partí»

Titoebefore, but has noc now. Wheréfore a Body which
has loft fome of its MoEion» by meecing Another» may
lofe more of it by a fecond Meeting, or a Third, and fo
on, ’cill ac lafl: it may be quice ílopped, as we ofcen
fee.

8. From whathasbeen faid, ít folio ws firít» thatif two
. like and unequal Bodies» be moved in a ftreight Line with

‘to‘mwe"hn- the famc Celerity, • the Greater Body ought to move longer
^er íhan lef- (han the lejfeT, l^caufe, the Quanticy of Motion in each
jíi- mts. Bodiesj is in proporción to their MaíTes» but they

communicate and lofe their Motion in proportiort to their
Superficies only, with which they ftrike againíí other Bo-
dies, amongft which they are moved ,• now though the
blgger Body has more Superficies than the Lepr, yet id
• has noc fo much in proporción toics Bulle» and confequent-
ly it does not lofe every Moment fo much of its Motiotj
as the kper one does.

9. One Inftance will malee this clear. Suppofe the Bo-
dy A to be a Cube two Foot every Way, and the Body

Tab.i.F;g. s- B5 a Cube of one Foot ; which being fuppofed, the Su-
perficies of the Body A will be four times as much as the
Superficies of che Body B, but the Mafs of it» will be
eighc times as big ; And confequently, ifchefeB.odiesmove
with the fame Celerity» the Body A will ha^ eight times

9. An Ex-
amjile.

7. The more Bodies there are of
a djííerent Magnitude, becween any
two- Bodies, fo much the greater will
the Veloclty of the Lafl: be : And it
wíH be the greaceílof all» M the Bo-
dies be in a concinued Proporción.
This eaíily folluws trom the pre-
ceeding Arcicle.

8., Perfe£Hy elaílick Bodies recede
from each ócher afeer Refle£tion,witb
the fame relative Velocuy» that they
approached each other v/ich befor^^
Refleélion i that is, in any givem
Time, the Difiance becween the two
Bodies before, and after their meec-
ing, will be the íame, ac the End of
that time. For the diftance of the
Bodies inany givendme, before they
meec, may be exprefled by a 4T b :
vU. thtí fame Quar.tir'es hy winch
the difference of their Velocicies, it
they be moved c! e íame way, or che
Sum of their Velocicies, U they he
moved diffL'reiicWaysjisreprefented ;
Alio the ¿paces which they deícribe

theQuantky - a jr B b tp Ab
^ A 4- B

which expreíTesthe Spacerun chrougfi

by the Body B afeer meeting, the fame
way chac A moved before meeting,
A a 2. B b ^ A D

be íubñraéled-

A 4 — B

which expreíTes the Spacerun througli
by rheBody A in the fame time, and
the fame w'ay ; the Remainder
Aa tp Ab +-Ba tp Bb _ , . ,

„ 4- b» wiH

A4— B ^

^ ■ ;,e,the Diflance of the two Bodies
ac'A ,Rnd of the given Time afeer
Reát^-^'n.

And like Reaíbning otlief

Laws may" fe- ijid.

I. Tlie ^ ousht to mová

lott^er') Icis tü be obK'.^.L. that chis
is fiid of Similar, that

Bodies. Ocherwiíe wearé^' nn-
derrtand by it, noc che Greateff J
the heavieíl Body : For the Motí^j
of Bodies that have the fame Celeri-

¡.nt, i Liiub lww .u.xjv. .v.»—

feparacely, in agivenTimeaafterRe- ' is noc as the Majfes of thofe Bo-

a.n - — L- n-.j i.-. .1 . t.... __ -l- .t.

üeíiion, may be exprefléd by the
fameQuancitieSjby which thefr Celeri-
^iesarc expreííed 5 whefeforejif from

but as che Weights of them.

See che Nms Chap. x. Are, 8,

as

53

Chap . II . of Natural Philosophy. .

aS much Motion as the Body B ; fo that it ought to loíe
cight times as much every Moment, in order for chem
to ceafe together. But this cannot be, becaufe the Body
A, having but four times as much Superficies as the other,
can meet with but four times as many Bodies, and not
with eight times as many ; wherefore the Body A will
move pretty quick, when the Body B will have no Mo-
tion at all, as is confirmed by Experience ; for if a Bullet
and a fraadl Shot come at the fame time out of a Gun,
the Bullet will be carried vaftly further than the fmall Shot.

lo, Secondly, Henee it follows alfo, That a long Body, ,o. That a
fuch as an Arrow, will continué to move longer, when it is BadywHh

tinne ío mwe

fiot lengthwife, than it would do if it went crofswije, for
it meets with fewer Bodies to transfer its Motion to, ¿tgoes tne
and therefore it keeps the more to it felf.

II. Thirdly> If a IBody moves alníofl wholly within it Zmthcr^°'^‘
felf, fo as to transfer very Jirtle of its Motion to the Bo- n. nata

» dies that furround it, it ought to contnme moving longeíi of ’*'*'/* -

,, . n • 1 r T 11 moves almoji

all : Thus we find by Experience, that a Iraooth v/ell po- h ftif,
lifhed Brafs Ball, of half a Foot Diameter, fupported by ««i*' y
two Pivots, will, wifh a fmall Stroke, continué to run
round for three or four Hours, ejiof aii.

12, But l^cauíé a Body cannot ib transfer its Motion to n. h»» »
another as to partake with that Body to which it is
transferred, b®^ill retain fome to it felf, though it be Vpholly
never fo lictle j therefore it íliould feem that a Body once Rtji.

¡n Motion, > iliould never afterwards be entirely at reft,
which js contrary to Experience. But we ought to con-
fider, that two Bodies which have but véry little Motion,
may be ib coimebted and adjufted to each other, as to
be in a mamser at Reíf, which is aU that Experience
íhows US.

13. Becaufe the World is full, a Body moving in a That a
ftreight Elne, muft of “ '

Third, but it ought nqt

ofthofe which are ^p^uíhad, wiU be forced to turn out /«i-s
of the Way, in j^lfto take the Place of that which was
firft mov^^^at being the only Place where they can ¡t¡ piaiee,

, go, andy nich is free for them ; Wherefore when any
Bojl^s moved, ^ a certain Quantity of Matter muft aE

fty puíh another, and that a ^“'6'
on thus infinitely ; for fome Zl,kfsTher

"l, Should never afterwards be en-
tlrely at Reft.) Thisisfalfe> becaoík
byiU upon a hlíe Foundation,
that Motion cannot be deftroyed».ffy,¿te
(he Notes above, Chap. x» Are, 1 3.

2. .A certain ^tantity of Matter.)
tjiis is foí (he moñ pait true; pot

E

becaufe the World ís fulla but becaufe
the State of the Air» and other Fluids
ín which Bodies are moved» is fuch».
that when any Body is moved ous
of its Place, thefe, by reaíbn of their
FluiditV} immed^acel^ run inep ihaz
Place*,

l

14 . That this
I^Qtion in a
CircUi ís the
Catife of ma^
ffy furpriz-ing
^Ivtions,

f y, Th& Ob-

f.nrityof the

jn^ords
itra¿lion,Syrji.
pathy and
]^nipathy.

RO H AUL’I‘’s PartJ.

ways neceffarily be moved in the Form of a Ring or a
Circle, or fome vj&y equivalenc chereto.

14. This Truth, though it was known long agOj yet
Philofophers, for want of duly attending to io and well
weighing and coníidering its Confequences, have thought
it impoffibleto accountfor all the Motions we fee in Na-
ture by Impufe alone, which is the only way that we
can conceive clearly, by-which one Body moves another
by pufhing it ^ and which fo naturally follows from thé
Impenetrability of Mateen which a!l the World agreein.
And this is the Reaíbn why they introduced inco their
Philofophy Things, indeed very Ipecious, íuchas Attra-
¿iio 7 í, Sympathy, Antipathy, the Tear of a Vacunm,
biit which, ac the Bottom, are mere Chimera’s, invent-
ed to make chem appear to give a Reafon of that which
they did not all underftand, and therefore ought not to
be ufed in the betcer fort of Natural Philofophy.

ly. For as to ' Attrañion-, Sympathy^ and Antipathyf
they ought not to be allowed at all, by reafon of their

Ob-

Since notbing a£ls
at a Diftdnce, chac iss nothing can
exertany Forcé in aólíng where i: is
noc i ifis evidenc, chac Bod'es (if
we would rpeak properly) cannot ac
a!I move one anocher, buc by Con-
tad and Impulíe. Wherefore At-
tra{ÍÍQ 7 t and áyrrípathy and all úccult
^Inalities, which are fuppored co ariíe
from che Specifick Fotms oi Things
are juílly co be rejeSed- Yet becaulej
"befídes innumerable other Phgenome-
na of Nacurcj that univeríál Gravita-
tion of ívlaccer, which íliallbe more
fully handled- afeerv/ards, can by no
means arife from che mutual Impulíe
of Bodies (becaufe all Impulíe niuíl
be in proporción to che Superficies
buc Gravicy is ahvays in proporción
ÍI0 che Quancicy of íolid Matter, and
í-herefore muft of Necellity be aícri-
bed co fume <'aure chac penecraces the
very inward Subifance it feltof fo-
jid Maccer) therefore all íiich Atira-
¿Ihv^ isbyall means to be allowed?
í!3 is not che Aftion of Matter ac a
Di flanee, huc the Aíiion of íbtne
immacerial Caufe which perpetualíy
/noves and góverns Matter by certain
Daws* Have not the fmall Particles
of 'Bodlei certain FocverS} Hirtítcs or
Forces, by which they afl at n djianesi
no: ordj itpcn the Rays of L.i'iht for 1'
rrfeélÜü^t refraCiing a?id ir.fe¿ling I
íke'rn'y biit atfü tifón ons atiatherfur I

pYodiicing a great parí of the Thano»
mena of Id atare ? For it is wellhnowtji
that Bodies a¿Í oneppon another by
the Attra£l:ions of (í^avity, Magni-
tifm and Eltüricit'* ¡ and thefe Ih'-
(iances fheto nonr and Coarfe of
Matare^ antTifial’e it not improbable
bíU that there may be more Actra-
¿live Püwers than thefe. Hoco thefe
AccraiStions may be performed^ l do
not here conjider. VFhat / cali Ac-
cra£tion may be performed by ImpuUé
(not Bodily Impulíej or by fome o-
ther Means unknoton to me, I nfe
that VFordherei to Jignify only in ge-
neral any Forcé by which Bodies
tmd towards one another, whatfoever
be the Canfe, For we mujl learn
the Phanomena ofNatnre, what
f one aJiother 7 and what

and Properties of the
^ hefore we mquire the
Canfe Accra< 5 líon is

performed, nf Gra-

vity, Magnetlfm a}j^^ ^ydty reach
to very fenjible DiJianceHfneffp, have
been obfervedby vtügar Eyesfa^' f^ere
may be others, which reach to fo
Dffiances as hitherto ejeape óbfer^pj^fffi
tion i and perhaps eleCirieal Accraíli-^
r n may reach to fnch fmall Dípat/-
H|¿ even without being excited by
Frft.'iion. Newc. Opc. p.

It fee?ns to me fariher7 that thefe
PfiYtkles (of Matter) have not only^

15

Chap. II. o/ N at.ura-l -Philosophy.

Obfcurity. That they are obfcure, is very evident j for
if we take a Loadftone, for Example, Ic ¡s manifeft to all
the Worldj that to fay it has an attraSlive Vertue or a
Sympathy with the Iron, does not at all explain the Na-
ture or the Properties Of it. And as to iht Fear of a Va-
cuum, I referve the Notion of That to the folíowing
Chapter, where we flrall compare the Reafoning of the
Antients and our own together.

a vis Inerti®, accompanied with fnch
pafjlve Lam of Motion^ ¿is natnrally
refnlí from thnt Forcé 3 Int alfa that
they are moved, by certain a£iive
Principies 3 fuch as is that (Attra-
¿tionwhichwe cali the Atcra 6 líoñ)
0/ Gravityy and that which canfes
Fervientationt and the Cohejion of
Jodies, Thsfe Princ'ples 1 conjidcr
not as occuJc Qualíties fnppofed to re-
fíilt from the Specifick Forms of
2hingS3 hiit as general Laxos of N^a-
Atures by vohich the Things themfelvts
are formedt Their Tritth appearing
íQ its by PhanomcYia thongh their
Caufes be not yet difcovcred, For
thefe are manifej? ^alities3 and their
Caufes only are occult, <And the
Ariítocelians ^ve the Isfame of oc-
culc Qualíties manifeji ^alitiesj
bnt to fiiCh ^m^ies only ás they
fuppafcd to lie and to

be the iinhnoton of manifcJl

Ejfeóls: Snch as voonld be the Caufes
of Gravityy and of magnetick, and
eleürick Attrañlons^ andof Fermen-
iationSj if we Jhould fuppofe that thefe
Fortes or AÜions aaofe from ^ta-
liiies nnknorvn to us» and uncapable
ot being difeovered and made maní-
fejl, Ssuh occuli Qualicies put a
Jiop to the Improvement cf natural
Phihfophy^ and therefore of late Tears
lave been rejecled. To teU us thau
every Speciei of Things is endojyf^á
voith an oceuk Specifick Qua)¿¿*S^
vfhich it aüs and produces^máfefi

EJfeóls, is to tell us mihing. Unt
to derive two or three general Prin-
cipies of Motion from Fhanomena, and
aftervoards to tell us. how the Pro-
perties and Aíiions of all corporeal
Things folloro from thofe manifeji
Principies, moíild be a very great Step
in Philofophy, thongh the Caufes of
thofe Principies were not yet dijeover-
ed : And therefore l fcruple not to
propofe the Principies of Motion a-
bove-me'ntionedi.3 they being of very
general Extenty and leave their Can...
fes to be found out. Id. Ibid^
p. 374.

— have the Authority of the
oldef and mofi celebrated Phílofophers
of Greece and Phcenicia, who made
a 7 acuum and Atcms3 and the Gra-
vity of AtomSi the firj} Principies
of their Philofophy ; tacitly attribn^
ting Gravicy to fome other Caufe than
denfe Matter. Later Philofophns
banif'} the CQ77fderations of fuch a
Caufe out of iiatural Philofophy, feig7}~
ing Hypothefcs for explaining allThings
mechanically, referring other Canjes
to Metaphyfichs. lí ‘heveas the niain
'Bufuefs of Namral Philof.phy is to
argüe from Phxnomena without fetg^i.-
ing Hypothcfcs , and to deduce Caufes
frotn EjJeCis, till voc come to the very
Firjl Lanfci rvhich certainly is not
^Mechanical, and 7ioí only to unfold
Wthe Mechanifin of the íVorld, hut
chiefy to refolve l'hsfe and fuch lil'S
1 §lllsfiions, &c. Id. Ibid. p. 343,

4

CHAP.

7

56 •

ROHJUL^'s Partí,

C H A P. XIL

Of fiich Motions as are commonlj afcriled to the Fear-
of a Vacuiim.

\.\What ttJrfi
CTiginally
meant by tht
Fiar of a

.Vacuurn*

T he RE is no Subjed more capable of íbowing m
the Difference betwixt true and falfe Philofophy, or
atleafl: betwixt Reafoning juñly and not juftly, than this,
P'or vve fee manifeflly, tfaat the one leads us, if not tq
the Truth, yet to fo great an Appearance of Truth, that
the Mind acquiefces in it ; but the other gives us only
Words, which we can form no Idea’s from. For Proof
of This, Let us take for inftance a Syringe, one End of
which being put into the W ater, and the Sticker drawn,
let US hear how the Antients reafoned about it. Firft, They
obfer ved, that tfiere could be no Vactmm in Nature ,• then they
coniidered, that there would be one, if the Sucker werq
drawfij and no Water followed; whence they concludéd,
that che Water pught to entcr in proportion the draw-
ing che Sucker j and henee they faid the 'FJw'x afeendedj
leíl there fliould be a

a. ñm the 2, Afterwards, the Manner of the Expfefflon was chang-
o/ th¡¡ wichout altering the Notion : and it was faid, that the

has baen cor- ^

VVatei afeended, íor fear there ftould be a Vacuimvo.
Nature ; And chis Expreííion being equivocal, it was ta-
ken in a bad Senfe j and as it is cuílomary ro carry Things
to Extretnicy, the VVord 'Bear was changed to Hgrrour’^
ib that it was affirmedj that the Water afeended, out of
the Horroiir which Nature had of a Vaettum, as if Nature
fin the Seníé that PhjlpfophS^tnderíiand that Word) was
capable of Horrour.

'3. The Bear of a Vacuum in ’^j^'atter Senfe, is very
ridiculous j wherefore I am apc tp thinlSíjí|-..the Phüofophers
ípokitin theformer Senfe only : But wiW¿j(i!tvay fo evey

it be underíloqd, it dpes by no means anftíi'tí^he, Que-
ifinn .• anv mnrp rhan ir w.'nnld. if anv nnp ílinii!d!í?K''ik nu,

iííon ,• any more than it would, if any one fhoulda'SKj.^ pw
Wood carne from very remore Parts to Perris, anv¿>
íhould be anfwered, it carne out of the Fear of Co!d-, tÉÜR
is no Anfwer to the Queftionn becaufe the fnal Caufe i?
ailedged inñcadpf the «¡ítífe, which was the Thing
áemanded. "

4. F(o,vy

Chap. 12. cf Naturai Philosophy, 57

4. However, if the Reafoning of the Antients v/cre
juft, and büilc upon a good Foundationj though it could
iiot make us underíland how the Water afcend?, thatis, ihe Fear ofs
explain to US the effcient Caule of fuch Afcentj v-icaum,daa
it íhould prove, at leaft, that it ought to afcend j and
their Reafoning íhould agree withExperience. And that Exicrma^,
you may fee that it is defecSive here alio, it is to be ob^
ferved; that if the fole Reaíbn, why any Space is filled,
is for fear there íhould be any Vacuum in Nature, and
this makes the Water afcend; as this Reafon is always
the fame, it will follow, that the Water ought always
to afcend, fo long as the Sucker of the Syringe is draw-?
ing, be it never fo long ; now Pumps being only long
Syringes, they ought to raife up Water to any Height
whatfoevér ; yet Experience íhews us, that we cannot by
Pumps, raiíe it above One and thirty Feet and a half,
after which, the Water ftops, and will not follow the
Sucker. Whence we ought to conclude, that the fear of

Vacuum, taken in the molí: favourable Senfe poffible,
is not at all the Caufe of the Waters afcending, fince it
does not agree with Experience.

5. Having feen the Defeéb of the Reafoning of the s- T'mom
Antients, la us fee if we can lay any Thing better found-

ed. And tl^I may not be guilty of the fame Fault, I thisanoihtr
íhall offer foa|^Particulars, which are very clear and
intelligible to alrme World, in order to draw fome cer-
tain and undoubted Confequences from a Foundation
which cannot be contefted.

6. Let US fuppafe firíl, That fome Body endeavours g. The
to draw the Sucker from the Bottom ot the Syringe

ABC, theHolIowof which it exadlyfits, that the whole “ ' ' *

Syringe is in the Air, and that the Hole C isopen : This
being fuppofed, it is evident, that the Sucker D cannot
be drawn towards E, but pulb the Air, which will '
pufh that beyond it, ’tifó|^was faid abo ve, it turns in
the Lines here defcrih^v' or fome fuch like, in order to
enrer into the whence the Sucker was drawn;

whence it feü^í^ :hat the Air was tnoved by a real

US fuppofe Secondiy, That the Hole at Csr-TheSecmt
ftopped, and that there were no Pores either in
Syringe or the Sucker; In this Cafe, Ifay, Htwould

?, h tottild he impojjlbh) T’; ,
woiild indeedbe true, if the Wóiíd
were full ; But becaufe wé have af-
íirmed it to be otherwife j fo much
foroe ojily is required to draw the

Sucker, as can Ufe the whole Weight
of che inciimbenc Air, Ñor need
we here trouble oiir ídves with any
occuii Poresor fubtile Matter,

5§ RO H JULT’s System Part L

be impoffible to draw rhe Sucícer, the leaft that can be,
becaufe ihe World being ful!, the Air which ought to
puflh the Suckerj would have no Place Cogo to.

S.TheThirii 8. Olí the othcr handj Let us fuppofc, that the Syringc
Suppüfition. thus ftopped, has Pores, though ío very fmalb asnotto
be perceived by our Senfes, and that amongít the Partióles
of the Air, there are fome fo fubtle, as to be able to en-
ter theíe Pores. This being fuppofed, there is no Reafon
why the Sucker tnay not be drawn, though the Hole ac
the Bottom of the Syringe be ftopped : For then the Suck-
er may tnake Room for it felf, by preffing the grofler parts
of the Air, and by fqueezing out the fubtle Parts, which
are forced to enter the Syringe.

Y- 9- order to know whether the Sucker of the Syringe

íf^urrefirlai Can be drawn when the Hole at the lower End is ftop-
:B¡¡dies havt ped we muft firft know, whether the Syringe or the
^íLTthTAir Sucícer haveauy Pores in them or no; and after that, whe-
confijis Iftwa ther there be any Partióles in the Air fubtil enough to
Sons ofFar- enter in at theíe Pores : For according to onc or other of ''
ticiss. tiieíe Suppoíitions, will the Thing be poílible or not pol-
fible. And becauíe neither of them can be determined
by our Senfes or by Reafon, and there being no Contra-
diftion in either, it muft be decided by E>:per'jence ; now
we find by Experience, that i if the Syrin^ be not too
thick, we can draw the Sucker without ,n Difficulty ;
from wheuce it is evident, that thereCe Pores either in
the Syringe, or in the Sucker, or rather in both of them ;
and that amongft the grofs parts of the Air, there are fome
fo fine, as to pafs through the Pores of raoft terreftrial
Bodies.

lo.- Anothcr lo. This Experiment helps us to another very coníider
T/birÉpni- '■able, which is, that if, after we have drawn the Sucker
ynnit; w a líttle, wc let it go again, it recurns of it felf, and that
'Js‘“ ^ Forcé, as to againft the Bottom of the

íswí.¿ary. tile Reafon of wbi&¿.;'i fliallfee, if we remem-

ber that a Body never begins to^.j^e of it felf, if it be
not pulEed by another which im^tdiately touches it;
now, if we obferve, that there is n^v¿z,,|)ut the Air,
that immediately touches the Sucker, weT¡S;}?^íhink that
it is the Air that caufes this furprizing Motion

j

f. 7fthe Syrin'!_e be not too thicl') i isj fo miich a Greater, and ccíHí^lj
Thé Ihicknefsof :he Syringe figni- j quehtiy fo much a heavier Colum¿Wf|
fies noíhíng (ñor che occulc Paflagesj¿* •>!•' Air muíl \i fuílain. Buc tbe Au-
nor the fubde Mactefí as was faid on^ v Ar may be excuíed, it he means
the yíriids above •,) büt chéThicknefs 'Bt^nrfs of ihe -whole Syringe.
tií chebncker j which che greater ic

fidering

59

Chap. 12. o/ N atural Philosophy.

fidering that the Air always contains in it a great Quanti-
ty ot the Parcicles of Water, and other terreñrial Bodies,
which though they be feparated from each other and dif-
perled, yet do noc lofe any of their Weight ; ( thoügh we
do not fully underíland the particular Nature of the Air,
ñor in whac its Weight confifts ;) 'Ctre íhall malee no Dif-
ficulty to aíTert; that the groíTer Air is heavy, and confe-
quently, that by its Weight, the Suckeris forced into the
Syringe, from whence it fqueezes ouj¡ the fubtil Matter
through thofe Pores which it felf entered in at.

II. But though the Air by its own Weight, preíles u.nMthe
chiefly donwards, yet this does not hinder, but that it
may alio prefs upwards, and forcé the Suckerof the in-
verted Syringe up into the Syringe ; for the Column of wards.

Air which anfwers to the Bottom of the Sucker, is for-
ced upwards [by the Weight of thofe Columns of Air
which are pn the lides, in the fame manner as the Water

which is at the Bottom of a heavy ladeo Boat, is preíTed up-
#wards againft the Bottom which refiñs it, by the W eight of
the Water which is of coníiderable Height round the Sides.

12. When we once underftand this Forcé of the Air to iz.wytoe

prefs upwards, we flvall not at all wonder, that when we * ,

hold our Hand fíat in the Air, we do not feel the ¡ncifm-
Weight of» that is, we do not perceive our Hand hm ^ir,
prefled dow^R^^^ by the Weight of the Column of Air

which is upon il'^For this Column has no more Forcé to
prefs it downwardsi than the Column which is underneath
has, to prefs it upwards.

13. As to the Preffure which ismade all over the Body, i^.whywt
when it is immerfedina heavy Liquid'3 it is certain, that donotfeeithe
we ought not to perceive it, i though the Weight of the °Ld
Liquid be very great, any more than we do the PreíTure «//» «-^7 Di-

vers Ho noü
feel the
Weight oj the

1 ^Thoftgh the Weight of the Ltc.0^'' ' Water.

very great) The Cauíe '

/Cxcellenrly well explaine^^^Jo,

Alph. Borellnij de a

Gravitare fea,

After he Sand in a

very cannoc any v/ay

be that a Wedge will

peans enter iniQ ic i and alio
acer in a Bladder» equally com-
íl'ed on ail Sides, can iieicher be
^reigheened ñor benc, ñor ac all nio-^
ved i So Ukewifc’, íays he> in the Iflc'v ^

4y of an AnhnaU there is contaiu'M
toithin the Skin^ fomc Parts^ which
^re hará and folidj fuch as the Pones j
ífthers that are foft> frsch as the Ten-

donSy Nerves^ Memhranes andMfíf-
des i and others that are Finida Wa^
try or Oily, Ffow the Pones in an
Animal cannot he broken or disjoint-
ed) tm'.cfsthe inettmbent Weight pref-
fes one Way only, as it does on íorters
Piit if the Prejftre difufes itfelfaU.
round., fo as to prefs upwards and
dovenroardsi and jldewaySi with e^ual
Forcé fo that there benopart ofthe Skin
hut what is prejfcd i then it is impojfhlei
that a7iy Thhig fmnid be feparated or
pTitoKt ofthe Way. The fame may be
filia of the Nerves and Mufles which
though they be foft^yet becaufe they con-
fjitf firong and tough Fibres, they can
allfnpport one another^ anáreffi an fí-
niysrjally

' ■ ROHAULr^s System Partí,

of the Water, when in divíng into the Sea, there are naa-
ny Fathom of ic over our Heads. The Reafon of which
is j that before we caá feel the Weight of any Body, there
muft be forne Alteration made in the Difpofition of our
Organs. But when the Air or Water have made all the
Efforts they are capable of to prefs or thruft inwards the
external and groílér Parts of our Body, and thefe Forces
are counterbalianced and put in ísq^tiil'thrio, by the Refiíl-
ance and Efíort o? the Fluids and moveable Parts within
US, the Ádlion of which we are infenfibie of ; after this,
I fay, they can do no more, and confequently the State
of our Body wül noe be changed, ñor the Diípoíition of
its Organs, to which they are fo uniformly applied, and
with fuch equal Forces, that no one lingle Part can move
outwards, to give way for any other to be thrañ inwards ¡
and therefore the Eífort whkh they continually make to
preís US inward, is rendred ineffeéiual.

umverfally dt£nfed fphartcal Compref-
^on ; fbe femé maj alfo be faid of the
‘Bloodand other Humours of an Ani
7'ríali which are of a voatyy Nature J
for^ as it is evidente that Water can-
Kot be condenfedi fo likexs'fe the Hu-
meurs of an animal, contained in the
Cavities of its Veffels, thongh they
ynay be bruifed by an Impulfe made
from one or a fes» particular Flacos,
yet 4hey can never be forced ont oj
their Veffels^ or torn afunder by an
siniverfai Comprejfon every Way, So
therefore as the SoUd, Tendinous^
err Flefhy, or Li^uld Parts, do not
títjder¿o any Separaúon, Contufon,
xor are disjolntcd, ñor their Situati’
en at all changed it is impoffhle,
that any Pain or Uneafnefs Jhould
follpw in the Animal^ which cannot
arife from any other Caufe, but fcpei''A
vating that which is one continuéis
7hiug, Wherefore when Divers, &c.

And chis is confijrmed by whac
tbe famous bAx.fBoyle obferved, in
his Second KAppendip: to the Elevenih
JHydrofatick Paradox^ viz. that a
Tadpole, an Animal whofé Fleíhís
very tender and fofe, put into a
VeíTe] half full of Wacer, fo clofed
'*dp, thac the Air contained in ¡c,
oeincr condení'ed eight times as much
asin ics natural Segre, preíTed upon
the Water a? muchj as if a Coiumn
of Water of Threehundred Feet in
Heighc laid upon the Animal 5 mo-
ved it felf notwithílanding, and
fwam about very quickj arad íound

no Inconvenience, that could be per-
ceived.

However, becaufe in moft Ani-
máis there is a greac deal of Air,
which may eafily be compreíTed and
condenfed j cherefor^iihough no par-
ticular Member is ^jointed, when
an Animal is iip/i-rfed very deep
in Water, muíl all of them

neceíTarily ícr lYreígbtned and con-
traíted, by the equal Weight and
Preflure oí the incumbenc Water on
all Sides, as the ¿mous Mr. Boyle
fays, bappened to the Tadpole in the
forementioned Experimgnt.

Befides, rhofe Animáis, vboíé
Lungs are ío formed as to contain
a great deal of thin Air and Breath
inthem, thnugh the other Parts of
them be not ai all burt, yet theiy
.Breafts muftof neceíTicy be ftreUhc-
¡t ■■•d and contra^íed, in the íarrie
ner, as the Cork is ufually thruí^
in'k^^Y^^mpty Bótele, by the Weighe
of cría!.** ,;fer, when it is íunk very
deep. -'re Men, whoíe Lungs

are very larg^.^«^n they dive very
deep into che Sea^> they find
no manner of Inconví^;’.-.- ' in any
other Part of their they.

labour iinder a BiíHculcy qf BvV .h-
ing, and a Pain in their
(chough they have Air enough corí¿J'i
'jVeyed to them to breache,) And chus
famous Mr. Üoyle tells us of a
cercain Diver, that when he walkcd
ac the Bottom of the Sea? che Blood
^ew ouc ac bis ^ofe and Eyes.

H-

Chap. 12. of Natural P HiLOsopair,.

14. Let US, in the fourth Place, Suppofe the Sucker
which is in the Syringe, as far as it can be thruft, co be
drawn when the Hole C at the Bottom is in the Water i
it íhould feem as if the Air which the Sucker that is
drawn preíTes upon, ought to preís upon the Water, and'
make it to rife in the Syringe, becaufe it overtakes it, in
the v/ay which we fuppofed it to go, in order for it felf to
enter in, if the End of the Syringe had been in the Air,
and not in the Water, and that it ought to afcend as far as
the Sucker is drawn. But there is no Neceffity that this
íhould always happen ; For having made it appear, thac
both the Syringe and the Sucker are full of Pores, and
íhat the Air is fuH of Matter, fubtle enough to país through
them ; and alió that the Water, by reafon of its Weighc
afcends with greater DifHculty j the Sucker may poífibly
be drawn, and the Water not neceíTarily afcend, to fill
the Syringe, becaufe it was filled before with that fubtil,
Matter, intermixed with the Air. However, Experiencc
%íhows US, that the Water does afcend, and that the Sy-
ringe is filled with ir, and not with the fubtle Matter,
at leaíl to the Height of One and thirty Feet and a half,
but no further. The Reafon of which is, that the Air
being heavy, preíTes upon the whole Superficies of the
Water in ®ich the End of the Syringe is immerfedj
and when th^S^ker is drawn, the Water which anfwers
to the Hole ih^^l End, not being preíTed by the incum-
bent Air, the Weight of that which preíTes upon the reíl
of the Surface, thruñs it up, and maltes it afcend in the
Syringe; in the íáme Manner, as the Water in a Pail is
made to afcend up a Trunk, fuch as tbey íhoot with, o-
pen at both Ends, and one End fixed in a Hole in a
Trencher which exaélly fits the whole Superficies; upon
depreíling the Trencher, the Water is forced up. In like
Manner, the Moving of th^jSucker, is the general Caufe
of the Entrance of fom^:^mer into the Place which it
leaves ; but the the Air determines the parti-

cular Matter.

ly. Since w^So^by Experience, that the Sucker of a
Syringem^^l^rawn, when the Hole at the End is ílop-
pedwí^^TuíEcient to conyince us, that the groíler Air
is^^' of an infinite Weight; for if it was, it would be
^®^offible to draw it ; which being fo, it is eafy tb forefee,
“That the Air by its Weight cannot raiíé the Water in a Sy-
ringe above a determinate-^^eight ; fo that if, after this
Height, we continué to afaw the Sucker, the Syringe, in-
-ítegd of being filled with Water wül be filled with fubtle
« ' Matter

6t

14. ítow «Sí
Water ¡s
drawn into
thi S)rÍKXíi

If- That thi
Water in a
Syringe ought
to rife bht to
a certain
Heightj and'
that a Co- ‘
lumn of Aht
xoeighs as
mxteh as a
Column of Ont
and thirty
Fcet and a
haff of IVai
ter of e^uai
Thkknefso

62

l6. That xoe
cnght not ío
^erccive the
IVcight of
the Air that
ii draxon inte
the Syringe ,
hiit w e OKght
zo perceive
that of the
Vi ater.

17 . When a
Tnbe filled
veith Water
cught to
empty it felf.

18 . That an
imlined Títbe
vtf^hí to con-
tal» more
Water than
el» iipright
ZTiCm

ROR AXJLT^ s StsTEM Partí.jl

Matter, as was before obferved in Pumps : And lince íJ
the Water always riles to about the Height of Thirty one 1
Feet and a half, above the Level in which the End of I
the Ptimps is immerfed, we ought to concludcj that a I
Column of Water of this Height, weighs as much as a p
Column of Air of equal Thickneís, which reaches to the
upper Surface where rhe groíTer Air terminares. y

id. If the Sucker of the Syringe ílips very eaíily along |
the concave Surface, againft which it rubs, and if it had
no Weight at all, the Air v/ould very eafily be drawn in;
becaufe there is juft as much Forcé to thruft it upwards,
as there is Weight upon the Sucker to thruft it down-
wards ; But if Water or any other heavy Liquor is to be
raifed ; there tnuft thén be as much Forcé uled, as is e-
qual to the Weight of the Liquor to be raifed ; becaufe
the Liquor, tending downward, bears upon the Air, which
preffes againft the Bottom of the Sucker, and takes ofF fo
much of the Forcé it had to make it rife. .)))

17. There may be many Confequences drawn from

what has been faid of the Syringe, which if they be a-
greeable to Exnerience, are fo many Confirmations of the
Truth of our Explication. For Proof hereof, let us fup-,
.pofe, for Example, that after having filled with

Water, one End of which is ftopped t^p'Ti'-the Matter
of which it is made (which they ihnelically feal-^
id) and the other, with the End of one’s Finger, we puc
the End of the Tube which is ftopped with our Fin-
ger into a VeíTel of Water, and then take our Finger
away ; This being; fuppofed, if we coníider that the Air,
which preíTes upon the Water in the VeíTel, reíifts the
defeent of that which is in the Tube, we may forefee,
that if the Tube be not above One and thirty Feet and
a half long, it will not etnpty it felf at all ; but if it be
longer, the Water ought t^>^-fcend till there is One and
thirty Feet and a half in die and then ftop, be-

caufe the Air has only Forcé enou^>? counterpoiíe fuch
a Qiiantity ; And this is agreeable to^’;”'-?,''nence.

18. We here fuppofe, that the Tubé?%^‘5bl,is above
Thirty one Feet and a half long is held uprighg^^.td. does
not incline one way or the other ; For if it incÍÍ^7ny j
way, then, becaufe the concave Surface of the T^™J
fuñains part of the Weight of the Water, for that Rea-™
fon, the Water will not hav( 0 .fp much Forcé to deícend
as it has ordinarily ; and fo me Air is able to fupporc a
greater Quantity than One áhd thirty Feet and a half in

the

Chap. 1 2. o/ N A T U R A L P H 1 L o S o P H y.

the Tube ; that is to íay, according to tbe J-a-as of Me~
chanicks, if the Water in the inclíned Tube uegínsto de-
ícend, it will ftop, whcn the upper Surtace of itj is One
and thirty Feet and a half perpendicularly above the Su-
perficies of the Water in the Veflel i and fo we find it
does.

ip. And it is remarkablej that if we make ufe of Tubes
of different thickneíTesj and Veffels of different breadth^
there is no diíFerence in riie Height of Water contained
in the Tubes : For fince the Water which isin eachTubcj
poíTeflés the Place of that Qiiantity of Air, which laid
uponthe fatnePart of the Superficies of the Water in the
VeíTel, it cannot but bein aq^uilihrio with the Air without,
becaufe, it weighs jufl: as much as that whofe Place it poF
feíles. And thus it is in all Tubes whatfoever, the Wa-
ter rifes to the fame height, which we.fee by Experience
in a particular Tube, that it ought to rife to j for as theíé
^ifFerent Colutnns of Water are of the fame height; if
^at, for Inftance, which is four times as thick as another,
weighs four times as much as that other ; then the Co-
lumn of Air, the Place of which this grofs Column of
Water poffeífes, weighs four times as much alfo.

20. NeitlKr ought we to findjany difference in the Height
of the Watí^Lvhich is in théTube, whether the Expe-
riment be m^jr^^the open Air, or in a Chamber, pro-
vided there be a*^indow in it, or at leaít any Chink
through which the Air can enter; for according to the
Laws of Mecha?!kks, the Weight of the Air is jufl: the
fame, whether it preíTes perpendicularly, or winding or
obliqué.

2t. Neither ought there to be any Difference in this
Height, if after the Experiment be made, the Room.be
entirely clofed up ; for though the Column cf Air v/hich
fupporced it before, by pre^^ upon the Liquor in the
Vellél, be now intercepí^^^ the Ceiling, yet that part
of the Column of A^%ich is below the Ceiling, preífes
as much upon thá^^uor as it did, when it bore rhe
Weight of t.te^^mof the Column, becaufe the Refiñ-
ance of_,^^^eiling does as it were prefs upon it, and
pS'irbm expanding it felf.

It is true, that if, before the Experiment be made,
dhamber be fo exadlly íhut up, that the Air within
ñas no Communication witb^that without, then the Li-
quor contained in the Tub ,' 'ought not to defeend quite fo
far; becaufe as the Tube empties, and the Liquor in the
Veflel rifes, the Air which is in the Chamber cannot rife in

Pro-

^3

the

fV/2ter

to he of e^uai
Height in
Tubes C'fMf-
fererit ThUk^
nefs.

20 . That
there willbe
"no xAlteraiion
wade in the
Height of the
Tf^ateTí ifthe
Experiment
be made in a
Place that is
Jhut iip,

21 . That the
Height of the
Water otight
iobetbefamCi
though the
P laceinxohkh
the Exper i-'
mentís made»
he intirely
clofed 7 ip,

22 . That the
Height of
the Water
ought to be
greateti ifthe
place bad
hecn entirely
clofed up bc^
fore the Ex-
periment was
made*

i^i^ckjthtr
úught not ío
tfmainm the
Tubii above
*he Height of
Twenty feven
Inches and a
*a^/.

¿4. ^af Ex-
j/eriments are
more éuJtLy
¡nade voith
^kh^vtr.

I5. T%dt
there is no
Vacuuni irt
tfíeTif of the
Tttbe»

z 6 . Another
^oof

ROHJULT’s Systzm Part Íí

Proportion : Confequently it muft be condenfed, and
therefore will have forcé enough to fuñain a little more
Liquor in the T ube ; but this canndc be perceived un*
leís it be a very little Place in whieh the Experiment
is made.

¿q. From what has been faid, it is eaíy to appre-
hend, that if inftead of Water^ any other Liquor that is
heavier or lighter be ufed, there will remain more ór leíí
of it intheTubej ib that Mercury or Quickfilver, which
is abdut fourteen times as heavy as Water, ought not to
be fuftained by the Air, but to about Seven and twenty
Inches and a half, which is very near a Fourteenth Pare
of the Height that Water is fuftained, and the reft of the
Tube, how long foever it be, ought to be filled witU
fubtle Matter. And this is confirmed by Experience.

24. But that the Experimenta may be more fenfible,
the Tubes íhould be made of Glafs, becaufe that is tranf-
parent ; And Quickfilver being ib heavy, that we are not
obliged to have Tubes much longer than Twenty feven®'^
Inches and a half, their Smallnefs maltes them more ealy
to be managed, and to obferve a great many particulars,
which it would be difficult to do in Tubes that are ve-
íylong. ^

2.5. Firft then, This may give Occafioiy'b thofe who
believe the PoGTibility of a Vacuum to^^-^ierve; That
there is no Vacuum i» the Top of the iMre 'i but the Place
which is left by the Mercury, is filled by fome Matter>
becaufe the viíibíe Objebts behind the Tube, * afíe¿t our
Eyes ftill, and are as plainly fenfible as they werc -before,
which they could not do, íf there were a Vacuum ; be-
caufe their Aiftion would be interrupted. And if the Eye
were placed diredily againft the Tube, we ought not to
fee any more than in the Dark, or than if an opake Bo-
dy were between ; but we^A”^ it otherwife.

26. To this we may ádd^waat i Ñothing or a Vacuuvf
fias no Properties, and that ñ^rjr^put the Top of the

T. our Syes) le don*c ac all

follow, fhat there is no Vacuum in
the Top of the Tube-, bficaun; che Space
oucüf which the Quickíilver carne,
is trdnfparenc i For wiiy cannoc the
l^aysotUghc, país thruugh an en-
tirelyvoid Syace} On che other Hand,
they can’c políibly pafs chrough a
Space that is (jititefnlt : See whac
is faid ot the Nacure óf Light, in
acs Place.

X. UQthin^^ or a Vacaum has no

tube

Vrapertits)‘C^:e.._ Vety trúe indeedl
that Nothmg hasNÍí.^^'rpperties i But
ho w dnes it followj'AiVryrace which
is void úf Matter, jrefore

Nathing in it, or is it feini";r.e'y
Núthing. Euc it may beallo'wS¿'''iC
there is fome finer Matter in
. Top of the Tube, or perhaps 3 littlfi* '

R lir flipt under the Quickfilver which'
rarefy’d by the Heat; but tb»
Space is very tarfiom beirg ful!.

Chap. 12. o/ Natural Philosophy.' 65

Tuhe very near tlie FirC:, we perceive a Rarefactiori:, in
thefame mannerj as in á Thermoraeter, whichtnakes the
Mércury ^all, whence ic follows, thac there is íbme real
Matter in ir.

27. However ic is cafy to fee thac thisSpaceis ilot ful! 27. That út
of common Air ; for if che Tube be not quite filled with '^°P "f
Quickfilver, but an Inch or two be left for Air, and ftop-

ping che End of che Tube wich our Fingerj, ic be inveit-
ed ; we obferve Chat che Quickfilver defcends ílowly^ and
we have time to fee the Air afcend in the Form of
Drops. Whereas let the Tube be entirely filled with
Qiiickfilver, and immerfed in the other Quickfilver, that ic
may empty it felfin the ordinary way ,• then if the Tube be
ílopped with theFinger and inverted^; the Quickfilver will
not fall ílowly, but all at once, as if it wcre onehard Body,
ñor íhall we perceive any Thing to afcend through it.

28. For a furcher Confirmación of chis Opiníonj mz. íS.nefhirH
That whén the ^kkjilver defcends from the Top of the í’'»"/-
Tube, it is not jtlled with common grof Air, we may obferve;
thátiftheTopoftheTube be madelarge, intheForm ofa

VeíTel, and fome Sort of Animáis, as BiMs, Rats,má Mice, hs
puc into ic, they will die, in a very Ihorc time ; that others,
luch as Ffes, feem to die, but being prefervcd afcerwards,
two or th^ Days in a more temperare Place they revive
• and fly aw^^g^ others, fuch as Worms and Brogs are prefer-
ved aíive, and^^hurt, unlefs they continué very long init:

2C). Ic may here be demanded, how the l'ubtil Mac- 29. VFhat
ter, which filis the Top of the Tube, gees through: To Pires thcjhi-
which it may be anfwered ; that it feems rather to país Zhlhhintkí
through the Pores of the Glafs, than thofeof theQuickfil- Top oftht
ver, becaufe the Quickfilver being very heavy, che Pores
- of ic feem to be rather too fmall for it to país through them :

Though I íliall be of another Opinión, if what I have
heard from England be tri?^ viz. s that a Tube of fix '
Foot long, will not e^^S^ic felf at all, if the Quickfii-

■>ér

A tong) This

iSs relaced by che

by

his Hydro‘

. i.fhat
Éxperimea*^'
hmo^c^^^'fVallis in

y^Trop. 15 , If the
Atfpended in an inverteñ Tube^ be
'■fy eaaíily cleared of all Air befare
' be inverted (which cannat be done
but bygreat Careand NiccnefSj] andJf
the Tube he.cautioufly invertedj JH
Jixed in a Jir7fi Place fo as not to he
in the leafl Jhahen} the ^dchjilver
(thongh the Orífice at the 'Bottom be
9pcn) will rcmam ffifpendídy mnch

ver

beyond the forefnentioned Height (víz.
to 40> íOj or 60 inehes j) bnt if the
leaji Air gets into the ^l^ickfitver
thiis fufpendédi or if the Tufe be flw-
ken, the ^luickfilver toill immediateiy
rjtfi] daron ta the nfnal Height (and
after fome ReciprocationS}') will fiand
JIUL

Which Ex'perimenr, having beon
ofeen repeaced by che Lord TÍrounkerp
the tamoiis Mr, Tjoyl, Mr. Hnygens
and others, has fiicceededi íb thac
there is no doubcof cercain Tnuii
of

RO H AUL'í’i System Partí.

ver wich which it is filled, and thac in which ic is im-
merfedj have ílopd forae time in a Place void of
grofs Air; For in iiiquiring inco the Reafcn of this
Fhxnomenon, we can find no other but thisj thac
the Qiúcklilver thus prepared, is cleared of fome

Macter,

of the PhíEnomencn ; buc opon whac
Caufes fo furprizing a Thing depends,
is not ícj well agreed.

The Lord 'Bronnker thought, that
the PYeight of the ^ir tViSts much
greater iban íinftoers to the Height of
abbut ¿9 Inchcs of ^^^ickfiheVy but
thac the ^tickfher xoas dcprcjfed to
that Heightj'by the ^ir vohitb was tn-
^'ifibly mixed voith it (tiulefs it was
eteared of it.) ^nd after it was
cleared ofiti and there rcmainedno^
thing to rtjijl the Weight of the ex-
terna/ ..Air} but only the bate weight
of the ^dcbjilverj then it was fottnd
to be otherwife 5 and the ^kkfilz'er
■ was fitppurted to a greater Heighty
hy the 'Ballance of the ..Air. This is
i ndeed very i ngenious 5 but thac v/hich
weakens very much chis Kxplícatíon,
is, thac lipón che leaft íliaking of che
Tube, the Quickfilver iinmedtacely
ruíhes down : which could by no
means be, If it were íupporced by
an eqiuil Weight of Air or Atchcr.

Wherelore the famousDr. P'P'alíis
attempeed che Thing another v/ay.
He imaginedj that all real Gravita-
iio»} proceeded fom the Pnjfnre or
Sprlng of the Air or t^/EihcYy without
which thofe inaÜive BoAies which
we cali heavy-, if once at rrjl, ivonld
remain fo^ withoíít''any real gravita-
úon} or without defeendingi havwgno
more Tcndency to mo-ve /^otv«iVízr¿¿í tha^
ftdeways, The ^Inicbjíl'ueY there fo\
when it is cleared of all Air from
woithin, andfnfpended in the aforcfaid
mantier ; when it is at rcjl-, will con-
tinué fo, andretain iís Pofttion} he~
yond the common Height necejfary to
an squilibrium ; becaufe it is free
from all 'Prrjfure of the .Airj and
is notprejfedupo»} either hy 'its Gra-
‘vity, or by its Spring ; But ij it be
puf in Motion, either by any Jlmking
of the TubC} or by any Motion whhiu,
from th^ Spring of the Air which was
at JhJi Uft in ií, oris fmee got i»} then
it will continué that Motion down-
svords (that way being open.)

Buc fince ic ¡s now allowedí that
Gravity docs not depend upon ihe
Aii or /Ether, buc is an original con-

nateand ímrniuable AfFeítion of all
Maccer, neicher can this Explicación
be admitted. And i ndeed chis very
learned Perfon confeflés, thac he him-
íelf was noc fatished wich ic, There-
fore he adds, That the Superficies of
the Tube however well polijhed, can-
not be thought to be fo free from all
Rnggedncfs or Ineqnality} but that
there mttjl remain fome Roughnefsj
which mufi canfe Cohafion-, or [if it
be moved) fome Friélion of the ad-
jace.nt Body, wherebythe Motion mufi
be fomething hindred.

And inóced chis Opinión comes^'
nearer che Truch i and thac chiefly
becaufe upon the leaft íhaking of che
Tube, the Quickfilver falls down;
whence ic is manifeñí thac che Suf-
penGon does noc depend upon any
permamenc Caufes Gp i as che Gra-
vity of che Air or^)zner, buc upon
fome accidencalTj^iig, fuch as fome
kind of Ad^gjjj^K.' fípwever, be-
caufe there doés noc appear to be any
fuch Roughnefs in the Superficies of
che Glafs, as chis learned Períon ima-
gines j ic feeins co be moñ proba-
ble, Chat che Quickfilver remains chus
üiípénóeá from the Contaél or Agree^
ment of the Parts, che Forcé of which
is always very greac in every Effeíl of
Nature. Thus, a plain and fmooth
Loadfiont applied to a Ball of Iron
fufpended on a Scring from a Nail,
“''—ill draw ic much further from che
¡ndicular» than in proporción co
tfl?*^netick Forcé j ific be pulled
bacK^É^ a gentle and even Hand,
and be^B^^paraced by any acci-
denta] ShaiS|||iSo alfo \Vacer will
afeerd in a fmall Glaís

Tubes open at boch'y!J^'''.,^And cwo
ímooch poíiíhed Mar^\¿Q‘'\\\ not
be feparaced, though the gror^’^.Air
be removed. And fo the Parcs\^'^Í
hard Bodics (and in fome meí
alio of Liquids) tobare together by
chatis, by thac .^ittraélion
wliich always arifes from Contaéf.
See what is faid bclow at Chap. zi.
Arde. 9.

All the Auíhor''s Pains therefore a-
bouc fubtil Matter, and abouc the
fores

Chap. 12. o/ Natural Philosophy.

Matteo which before kept its Parts at a Diftance. and
made the Pores fufficiently wide and long, to give ’free
Paflage to the fubtil Matcer j and becaufe it cannot thruft
the fubtil MaCter into the Place which it is difpofed by
its Weight to quit, therefore it does not defcend at all ;

However, not having had any Opportunity to fee how
well this Experiment fucceeds, and not venturing to fay
that it is falfe, we remain in fuípenfe, and do not deter-
mine which Body it is, through the Pores of which the
fubtil Matter paíTes, to fill the Top of the Tube.

30. But to return to our Difcourfe, and to continué 50. wuat
to draw the Confequences which wethinkdeducible from

what has been faid above ^ Let us fuppofe a Tube filled Te"ií al
with Quickfilver, and immerfed asufual in a Veffeh in- Tabe be Ufieá
to which Part of the Liquid runs, ’dll it is about the HTim.
Height of Twenty feven Inches and a half. and then it be er End of it
liftedupalittleabove theSurfaceof the Quickfilver, fo that ^ ^

one Drop only of it may run out ,• then becaufe the Quick-
filver, that remains inthe Tube, does not weigh fo much
as the Air without, it ought to be impelled with Violence
to the Top of the Tube, and after that, its own Weight
ought to make it defcend on the one hand, as much
as the Ailjtoakes it afcend on the other 3 and fo we find
it does.

31. If, al^^aving made the Experiment as ufual, we 31. That »»

take the Tube out of the Veffel in which iris immerfed, “

ftopping thelower Hole with ourFinger, but not

very hard upon it, then we ought not to feel, ñor do we the
indeed feel the Weight ofthe ^ickfilver; For though it veraatum
lies upon that part of the Finger, which anfwers to the
Hole of the Tube, yet it is not heavy, becaufe it preñes
neither more ñor lefs, than the external furrounding Air,
which is applied to the other Part of the Finger, preñes
upon it, and repels it. JjM if in this Cafe, the Tube
be opened at the by fuddenly ' femoving that

thi Tühe.

Pores through whi¿|ÉBir fiílitíoiis
. Matcer íhoul^^aÉPKo no Purpoíe.
For if PaQáge for that

fuht^^á^Kri e’uher through che
or the Glafs ; yet ic
ÉHld not be able lo forcé che
mf^ickjilvé^ up into the Tube, ñor
^o íúñain ic there': And if there be
no Paflágq for ic througft elrher of
them, then ic would noc íuíle/')he
Quickfilv¿ to fnbfide again, ás ic
does when che Glafs is fhaken. But
indeed ^he Parcicles of ¡i^ckjther,

' F 2

when ic is firíl cleared of all Air^
cohscre by mucual Concaíi, boch with
oneanorher and wich the Glafs i/rom
a certaí» Aitraümh which ceafes, as
füon as che Tube is íhaken, whereby
che Parcicles are feparaced from cacti
other, and from the Glaís. And the
íame Experiment has been made in
.\Vacer well clear’d .alíb of Air, hy
.which means its Parts approached
ncarer; to Conca¿f. Stt Newt. Op-
tích,pa¿. 337.

which

6t

ROHJULT>s System Partí.

32. VFhat
Might to be
the Confe~
tjttence of jil-
ling up the
Tiibe voith rf-
ny otker Li^-
^nar, .

33. And.
vohat, ifit be
jilled voith
Air,

^^.That the
iffeéis of Air
are different
Aícerding to
the di^erent
Lengthi of
the Tules.

%$, A very
good Experi-
ment of a
Carpas ^Blad~
deti to Jhow
hoto mtích the
Air is capa*
bit of ex*
panding. it

whidi it is ftopped withj then we íliould feel the fame as
if the Finger which is appUed to the lower Hole received
a hard Blow j becaufe che groíTer Air, which defcends
quick, and with great Forcé hito the Tube, adds on a
fudden new Weighc to that of che Quickfilver ¡ and chis
is confirmed by Experience.

32. If theTube be not filled with all Quickfilver^ but
fome other Liquor be puc in alfo, we may determine
how far each of them oughc to defcend, by confidering
how much that other weighs compared with the Quick-
filver. For Inftance ; Suppofe theTube 'filled with Quick-
filver all but an Inch, and we would fill the reft with
Water 3 becaufe Water weighs but a fourteenth Part fo
much as Qtiickfilver, we ought to conclude, that it will
make it defcend below the ordinary Station, the four-
teenth Part of an Inch, and coníequently the Water will
be Thirteen of the fourteen Parts above that Station.

33. The like Calculación may be made, whatever hea- #
vy Liquor be put in inftead of Water ; However, it is

to be obferved, that the fame Reafon will not hold good
for grojf Air. For fiiice we know by Experience, that
it has a Power of expanding it felf very much and cari
eafily be mixed with che fubcil Matter 3 we c^^ceive that
by mixing it felf with that fine Matter ■w|.’f:»which the
Top of the Tube is filled, it preffes agGíilt the Top of
the Tube on the one Part, and upon the Top of the
Quickfilver on che other Part, and fo by chis means for-
cea it much lower than it would forcé it by its ówn
Weight, which compared with Quickfilver bearsno pro-
porción to it.

34. We forefee alfo, that an Inch of Air will make the
Quickfilver defcend fo much the lower, by how much lels
the Tube exceeds Twent yj^ny en Inches and a half in
Length, becaufe the Power CTlj^ting it felf, does in a
manner refemble a Spring : fOT^^a Spring, the more
it is bent, with fo much greater FOTÍj^oes it unbend it
felf 3 fo the Air, the more it is compreR^^wth fo much
the greater Forcé does it dilate it felf 3 anoa^.W^his, our
Reafoning is confirmed by Experience.

35. But to give a plainer Proof • how much a ,

Air, when the Weight of theColumn which it fufl:ainshi^|J
removed, is capable of expanding it felf 3 we need oniy “
take a Carp’s Bladder, and cutci% oíF the leffer Pare at che

I. ff/jw much a little Air') See ] Ast. 3. heUra.
the Notes on Part III. cha^, 2.

Ncckj

Chap. 12. N ATURAL Philosophy,

Neck, where it is joined to the Greater, prefs. the greater
Pare fo clofej as to fqueeze out almoft all the Air thát is
contained in it : Then tie it up to keep in thaLwhich re-
mains, whichis not bigger than a fmall Lentih^fter this,
let it be put into the Top of one of the Tubes made
large like a VeíTeh and Hlled as ufual with Quickhlver,
and managed in the fame manner as the forementioned
ExperimentSj and then we (hall fee how furprizingly the
Bladder will íwell round almoft all at once, and appear
to be blown as big as it was befare che Air was let
out.

3Ó'. Now though there be much more fubtil Matter 36.
in the Bladder thus diftended, than grofs Air ; yet we are
not to think, that it is that which preíTes upon the Ínter- theoTiailtin
nal Parts of the Bladder, and fwells it thus 3 this Effed can-
not be produced by it, becaufe it can eaflly-return through
the Pores by which it entered 3 it is more likely, i that
this fine Matter agitares that little grofs Air which remains
in the Bladder with great Violence, which Agitation is the
immediate Caufe of the Bladder’s Iwelling : And this is fuf-
ficiently evident ; for if the Bladder be entirely emptied of
the grofs Air, it will not fwell at all, and if there be a little
too mudL it will break.

37. Inwder to make this Experiment well, it íhould 37*
be done x^^^Jube open at both Ends, and the upper

End íhould be^overed with a Hog s-bladderj moiftned this Ex^sú.
firft in Water, that it may ftretch the better, and this
will give us opportunity of obíérving andther Gir-cum-
ftance very curious 3 and that is, that as foon as the Quick-
filver begins to defeend, we íliall fee the Hogs»Bladder
ftretched, and forced into theTube^ the reafon of which
is3 that then a very heavy Column of Air preífes upon
it, and there is none under it to fupport it.

38. Ifche Bladder be p^l^d with a Needle, and theNee- 38. ^nothir
dle be pulled out a lin^j^o let fome of the grofs Air in,

and then the Hole^iRopped ; the grofs Air which enters
in, will exparrdJpjm round the Carp’s-Bladder, and prefs
upon it, agp^te it appear more or lefs wrinkled, ac-
cordioalÉl^he Quantity of Air let in.

his Experiment may ferve to undeceive thofe, 39- The üfe.
mo upon reading Arifiotle have been of Opinión, that
^jíir made ten times rarér than it is> neceffarily ohanges its
Natureyandis canvertedinti^Fire. For the Falíity of this Ima-

I. That this fine Matter) Not 1 no fuch Thing, biit only tlie ElaftU
thít Witter, for probabJy there is 1 city of the Air it felf.

F3

ginatioii

^<3 RO H JUL'T*s Svs'ü'EU Partí.

gination is clearly feen, by íhowing that the Air contained
in the Carp’s-Bladder is rarifyed above a hundred Timesj
and yet dais not at all alter its Fortn.

40. That the 40. Whcn I fpoke of the Height which the QHickfilvei:

0/ the ftands at in the Tube^ I limited it to Twenty íeven Inches
wrhnÍT’^ ” 2nd a halfj which is the common Height obferved at
Parts i but to fpeak exaítly, it is fotnetimes higher, and
fometimes lower.j becaufe the Air at diflFerent times is
lighter and heavier.

41. That the 41. One of the beft Obfervations that I have met wich

TJ/k‘n!t’tí Subjedt is this : That though we know by Ex-

aiter the períencc, that the Air is condenfed by Cold^ yet I have
^ei^ht of the never found that the greatefl Cold, made any Alteración
midwhZ 7 the '^^‘2 Height of theQuickfilver in the Tube. The Rea-
Caufssare, lonof whichj io my Opinión, is, that the Coid being ve-
á/í 7 r ^ Superficies of

the Eartb, the Air does not paE from one Country to a-
nother ib that the Bulk or Quantity of it is increa- 0
fed ; but it being condenfed only from the Top to
the Boccom, it is the fame Quantity of Air, that preííes
upon any particular Place of the Earth ; lo that all the
Difference that there can arife in the Air, muft be impu-
ted to more or leE ‘ Vapours and Exhalations,wh/h are con- *■

’■ tained in it at differeut Seafons, and to the Ví^ods which

blow fometimes upwards and fometimeagííwnwards.

' ' '42. As

1. VapouYs and Exhalations) Ic ,
has been long obferved, thac in clofe
and rainy Weacher, che Quickfilver
does non rife ío bigha as when ic is
dry and clear'5 which has been
rhoughc by fome to overcbrow che
whole Theory of theWeighcof the
Air ; and indeed itisvery difficulc, to
explain parctcularly the Caufes of ali
the various and minute Changes of
the Ideavens; a great deal is owíng
tó the Winds, which blow fomecimes
upwards, fometimesdownwards, and
íometimes íideways ; a great deal to
Vapours j a grcac deal co Stearns ri-
fing üut of ihc Earth j íomething muft
be afcribed to the Alteración of the
Heavens in the neighbonring Coun-
tries,' and perhaps íomething co that
.rinx and Rejiiix which the Moon
caufes in the Air, which is much
greater than that in the Sea, ó-c, To
accüunt íor all which particularly
and cxa£tly, wouldbeendlefs. How-
evcr, to propofe fomeihing which
may come pretty near the Truth j ir
ifi co be obferved, that the Airit felj

is heavier than the Vapours, and fít-
ced có íúpporc them, becaufe its
Parcicles are groífer, and arife from
denfer Bodies, than the Parcicles of
Vapours.

In che firft Place therefore, this
Weight of che Air, in any particular
¿puntryj may bé fo changed by the
Atmofphere may
oW^^eriíéd and made heavier, by
brin|^|^ greater Quantity of Air,
and heapBjc;>c togecher ; viz. when-
ever cwo^(^>h; blow at che fama
time from Parts of the

Heavens ; or íoméí¿'\he Air may
be carried or blown íi??íjT‘-'',,them,
andchereby an Opporcunit^i..¿. 7 :--,ta
che Atmoíphere ro un^old íc felf,'.*-'V,
incumbenc Weight being taken
■viz. as ofcen as cwo Winds blo\\^®jf|
from che fame Country to oppofice
of che Pleavensj or whenfbe-
ver any one parricylar Wind is very
ftrong j for ic is found by Experience,
thac an artificial fti\;)ng Wind makes
che Air lighter, and^che Quickfilver
in che Tuoe to fall vei^y much. Seé
í&é

of Natural Philosophy.

71

42 . That mi-

Chap. 12 .

¿¡2. As to any Alterarioh in the Height of the Quickfil-
vei-j which may be thought to arife from che Dilatación of
the fubtil Matter in the Top of che TubCj by the Heat mcr, ñor the
of the Summer, or the Concraction of it by che Coid of
the Wincer, ic cannoc be at all feníible ; For Experience Jifeufíiydi-
fhows usj that if chis Matter be heated by a Fire, much latear cm-
more than ic can be by the Heat of the Sun, ic will noc

the Tabeo

the PhilofophlcalTranfaóiionSi lHitmb.

Secondlyj Coid and nitroas Parti-
tles of the condenfed by

Coid f*om the North muft condenfe
the Atmofphere where-ever it comes,
and make ic heavier.

Thirdly, Heavy and dry Exhala -
tions make the Air heavy (^in the
fame manner as the Speciñck Gra-
vityof any Menjirimmh increafed,
by diílblving Sales and Metals) and
nsclajiick Forcé, as íc is called, meft
ihereby become fo much che fírong-
er.

Fourthly, When the Alrby chefe
and fuch like Caufes is become heavy,
rhen is ic more able to fupporc che
Vapoiirs s which when chey are en-
cirely mix^nth ic, andfwim about,
and are ewy way difperfed in ir,
make che S^^ferene and clear : Buc
when che the concrary

Caufes, is má^Hjtóter, then is ic
unable to fupporc tne Vapours with
which ic is always hlled, and fo be-
ing puc inco fome forc of violenc A-
gitacion, chey gacher chemfelves into
Clouds and Mifis, and being formad
into Drops, fall down.

From chefe Obfervacions, ic is ve-
ry evidenc, that che fame Caufes,
which makecheAir heavier, and more
able to fuñain che Qnickfilver in che
Tube, make the Heavens alfo clm
gnd dry ^ and by the fame Gauí^B
which rhe Ah* is made lighfJj^^d
lefs able to fuñain the (^j^^lver,
are Showers and iyáj|^produced
alío.

Henee icfoll||||^ffirflj Thacwben
th« Air Js^jjpBjr and the Quickül-
verfa^j^efl in the Tube, then the
fij||PKiove very low and quick j
jjjRathat clear Air, which afeer Rain,
j^^ppears becween the thick Clouds-,
^ being diícbarged of ics Vapours, feems
moft cranfparentand bright, andgives
the beñ and ealieft profpedt of /'kings
at a diílance,

Secondly, When the Air is more
heavy j and che C^ickíilver is raifed

F

higher in che Tube, then che Hea-
vens are fair, buc a lítele thicker, and
not quite fo blue, by reafon of chfe
Vapours which are every way equal-
ly difperíéd about; and as has been
by many obferved, ic doesno: aíFord
ib good a Profpe£t of Things ac a
diílance ; and if chere do appear any
Clouds, chey are very high and move .
very ílow ; and when che Air is
heavieíl of all, the Earch is íbme-
times covered wich very thick Clouds,
which feem to conlift of heavier fbre
of Exhalations, which the Air ac that
time is capable of íiiílaining, buc
which cannoc fwim in lighier
Air.

Thirdly, Henee it is, that in our
own Counery, when the Coid is grear-
eít, and the Nordi and North Eaíl
VVindsbíow, ihe Quickíilver in tfie
Tube is higbeñ; becauíe ac that time
two Winds blow cogetherupon our
Country from oppoíite Parts of che
Heavens ; for in the Atlamick Oce-
ah, accheíame E^f/VK^£Wich us, the
Wind blov/s almoft always from the
Weíl. To which we may add, that
the Air which is brougbc hither by
the North V/ind, comes condeníéd
by the Coid.

Fourthly, In the moñ Northern
Councries, chere is greacer Variación
of tile Heighc ot ché Quickíilver in
che Tube, than in thoíe Councries
which are more South, becaufe in
chufe Councries, che Windsareílrong-
er and more variable 3 and oppoíed
by each other in a leís Tracl ot
Land i wbence che A/r is fbmerimes
more heaped np. and condenfed, and
fomedmes carried away and lighc-
ned. ‘

Laííly, Becween the Tropicks,
chere is che ieaíl Variación of ail, in
che Heighc of che quickíilver in che
Tube, beca-jfe chere the Wind is tor
the moíl pare very gentla, and blow^
che fame way.

See the Philofopklcal Tranfiñions^
Ntimbcr 181 »

, 72

43 •

much the
greateji Díf-
ference in the
Height of the
^uickjtlver

44. Tbat the
Height of the
^Inickjther
onght to he
df^erent in
places of a
dijferent
iidghu

AS'

Éx^criment,

46 . Anothtr
mon- JsuJibié
Ex^eumsnt.

^0 H AÜ L'T’s ^STEM Part I.

make the Quickfilver defcend at all j and if the Heat of
Summer can do noching towards feníibly dilating it, the
Coid of the Winter can much lefs do any Thing towards
the condenfing it.

43. But whatever be the Caufe of the Quickíilyer-s r¡-
íingand falling in aTube, where the Experiment is con^
tinual ,• the greareft Height that I have obferved for fif- .
teen Years, in a Tube which I prepared for that Purpofe,
was Twenty eight luches, and a third Part of an Inch ;
and the loweft was Twenty fix luches and feven tweifth
Parts of an Inch ,• fo that the greatefl. DifFerence in the
Height of the Quickfilver, was an Inch and three quar-
ters.

. 44. Though all thefe Experiments are fuflficient to con-
vince US, that it is by the Weight of the Air, that the
Water or Quickfilver is fupported or made tq rife iti the
Tubej yet it is eafy to conceive how there may be an
Alteration made in the Height of the Quickfilver, and yet f
no Change made in the Air it felf ; In order to this, we
need only make the Experiment in two diíFerent Placesi
the one the higheft, and the other the loweft that we can
come‘at ; For there being a lefs Qiiantity of heay.y Air iri
the higheft Place, the Quickfilver cannot be íürforted by
it to fo great a Height as in the loweft.

45- Now in ordeno tryifExperience ^< 111 !^ agreewith
our Reaíoning, I filled a Tube three Foot and a half long,
with Quickfilver, and immerled it into a deep and ftrait
VeíTel, into which it emptied 'it felf asufualj after which
I fixed them both in a Wooden Erame, made for that
Purpofe ; And now the Inftrument being fuch as could
conveniently be carried from one Place to another, withf
out any Danger of fpilling ; I carried it to the Surface of
the River Seine, which happ^ed then to be frozen, and
obíérved exadtly the Height of^fcMercury : After which,

I went up one of the Towersof ln|^hurch of the Virgin
Marjal Fíiris, which is about Two^É^red and fixteen
Feet higher than the Place where theTli|^^xperiment
v/as made,- and here I found the QuickfilvSl^^not fo
high in the Tube as before, by near three Line^j^retcis,
ncar a quarter of an Inch. 'tyv_

46'. The fame Experiment was tried in inone'^h

ofthe loweft Places of theTown oí Clermoiit, and upon
the Top of a neighbouring MoLUjtain, calledT»j' de Dome:,
which is about Three thoufand'Eoot higher than the Val-
ly, and the DifFerence in the Height of the Quickfilver was
found to be above three Inches.

47 * -

Chap. 12. of Natural Philosophy, yg ,

47. As this Experiment is more fenfiblethan mine, if 47 r-^
t was made, as there is Reafon to think ir was, wich all

he ExaclneE one coilld wifh ,• ir furniilies us wich an 0/ the AH.
eafy Method of finding the Height of the whole Air^, íup-
pofing it to be every where of che fame Denfity as ic is
lear the Earth ; For fince upon taking away Three thou-
íand Feec of Air, the Qnickfilver íinks three Inches, chis
¡s a Pr(Jbf, thac a Column of Quickiilver of three ínches
high, weighs equal to Three choufand Feec of Air, and
confequeritly the Height of the whole Air, which coun-
terpoifes Twency feven Inches and a half of Quickfilver,
is' Twenty feven thoufand and five hundred Feec high.

48. As therefore we conclude, that when there is leís 48. That all
Height of che groíTer Air to prefs upon the Quickfilver

inthe VeíTelj there ought alfo to be lefs Height of thzt yaii omof the
in the Tube; for the íame Reafon, if we fuppofe that T«be,ií there
there were no grofs Air ac all to prefs ic upwards, we tJ’pfe/i
7|ghc to conclude that all che Quickfilver would fall down, ttpm the Vcf-
fo that That in the Tube would be level wich that in the
VeíTel.

49. Some llave imagined it impoíTible to make any 49; De-
Obfervation by which it Ihould appear, that Reafon and
Experience aa^e in this Particular ; becaufe enere is no malte Ms
Mountain hi^jenough to carry us up to the upper Sur- Experiment.
face of the Air / ‘ts^becaufe, if there were, the Air would

be ib chin, that wecould not breathe in ic. But I thoughc

of a Means to remove thefe two Difficulties, and by

which the Thing might eafily be eíFeded j and that was,

to prepare fome fmall Room, with tranfparenc Walls,

which one might ftand without and look upon, without

any Danger from what might happen within. I caufed

therefore a Glafs Inftrumenc to be made, according to the

folio wing Reprefentacion. BC)i*is a Tube, upwards of Tab.i.Fig.y.

Twency feven Inches and a.d'j^.-.'long, and is open at C:

AB is a large Cavity, wlji^v^* has a Communicacion with
BC by the Pare BL, qh^ is clofed, and has no Aperture
at A ; DE is a^íf!;yt&'- Glafs Tube ftopped up at the End
D, and ílicte'Sfe’^f the Cavity AB by the Length FE,
and is : Befides there is a fmall Hole F in this

little ^abe, where it is cemented on the outfide to tha
in fuch a manner, thac the Cavity of the little
^^oe has a Commupication, with the large Cavity AB
JTby this little Hole F : Laftly, by means of the Neck BG,

I the external Air has a Communicacion with that in the
I whole Tube A,BC.

RO H AU LT’s System

Part 1 1

74

50. Hm tht ^o. I fii'ft ftop the Hole G with a Hog’s-Bladder, and
JSmiíftó turning the whole Inftrutnent, fo that the End C may be
ietife4. upperraoft, then I pourin the Quickfilver at the Hole E,
which at firft falls only into the little Tube DFE, buc
wheti it is full up to Fj then continuing ítill to pour in,
it runs through the Hole there, and filis the Cavity AB
which furrounds thisTube, which I fill up as high as B ;
then I fill the reft of tlielarge Cavity^ pouring the -Quick-
íilvec in at C, ’till it rifes as far as the Hole E, which I
ftop then with a Hog’s-Bladder ,• after this, I continué to
pour the Quickfilver in at the Hole C, ’till the Tube BC
is quite full ; Having done this, I ftop the Hole C with
rny Finger, and inverc the whole Inftrutnent which is full
of Quickfilver as ufuah and itnmerfe it in a VeíTel of the
fatne; Then the Cavity AF empties itfelf as far asIL, and
at the fatne Time, the little Tube DFE empties itfelf to
the fame Height, and the Tube C empties it felf to H
which is Tweutyfeven loches and a half above the Qui<^-
filver in the VeíTel ■. And thus we fee that Reafon and Ex-
perience agree : for as there is no grofs Air to prefs upon
the Surface IL of the Quickfilver whích^remains in the
Bafon IFL, fo there is nothing to forcé it to rife in the
little Tube DFE. £

which_^ops the H0I5

yi; Surprt- 51 . Now if the Hog’s-Bladder

at G, be pricked with a Needle, ii^íívident, that the

franc'élf^ths gtoffer Air which enters into the Cavity ABG ought te
uíir into the produce Two very difíérent, and therefore very remark-
inflrnmenu Effeifts : The firft is, That preffing upon the Quick
filver which is diredily under G, it will caufe it to de-
feend ; and alio preffing upon theSurface IL of the Quick-
filver w'hich remains in the Bafon IFL, it will make Pan
of it to afeend in the little Tube DFE, and fill it quite
full, provided it does not exceed Twenty feven Inche

and a half in Length. Hfcj^xperiment will be more plea-
fant, if after the Hog’s-Blaoí^, with which the Hole G
is ftopped, be pricked, the r^l^dje be pulled back feve-
ral times a very litde, to let a littraíj||^H at a time through
the Hole, and then thruft forwardqS^^ jt again ; fot
then you will have the Pleafurc to fee th^'’^;;v'(cfilver in
the little T ube DFE afeend by little and littTcr’.^.the fe-
veral times, and that in the Tube BC defeend in tii
Then if the Needle be pulíed out all at

you

manner.

Chap. 12. <?/Naturál Philosophy.

you will fee at the fame time it will rife as rriuch on the
eme Hand, as it falls on the other. r

52. If the Liquor with which the Bafon belonging to
the Tube is filled^ falls all down, becaufe there is no Air
to fupport it, as wefeein theforegoingExperiment, where
the little Tube DFE, is entirely emptied of the ^ick-
filver ; the Reafon holds ftronger for its not riling, if
there be no Air to thruft it up j wherefore there is no
need of máking any Experimenc, to be affured, tbat the
Water ought not to rife in a Syringe, when the Sucker is
drawn, if the Veífel in which the End of the Syringe is
immerfed, be fo ftopped, that the external Aircannot en-
ter into it. But if any one be ftill fo obftinate, as not to
be content without referring it to Experience, he need
only put the End of the Syringe into the Mouth of a
Glafs Bottle, which is round and ftrong, and full of
Water ,■ but not begin to dravv the Sucker, ’till the Mouth
TÍ the Bottle be well ftopped wich Wax, or fome fuch
Thing, to prevent the external Air entring ; and then
he will fee that the Water will not rife at all in the Sy-
ringe.

53. That^e may go on to explain the moíl coníidera-
hle Phíenome^oí Hydraulick Inftruments ,• I come now
to give an A^j.unt of the Syphon. Let ABCD then be
fuch a Syphon, fríí^firorter Arm of which CD is put in-
to a Veffel of Water ; Then, as has been often faid, the
Air which preííes upon the Water which is in the Vef-
fel, ought not to make it rife up in the Syphon, becaufe
the Air which is in the Syphon hinders it.

54. But if the Water in the Veífel be made to rife up
into the Syphon, either by fucking it at the End A, or
any other way, fo that it be filled quite full of Water,
and then we take our Mouth from the Hole A, the
Water will not ceafe to r^r'j^-'but continué running, fo
long as the íhorter Arra ^O' remains in the Water in the
Vellel : The Reafoi?,!?:^/ which is this. So long as the
íhorter Arm Cp<^Srimerfed in the Water, the Forcé of
the Air •ind^|'^’i?-%hich preííes upon -. the Water in the Vef-
fel, ^‘¥.l5í,gch endeavours to make it rife in this Arm, is
not^^fiSly greater or leís, than the Forcé of the Air

endeavours to repelí it, when it ofFérs to run out
íit the Hole in the other Arm ; But becaufe the Forcé of

75

jz. That the
ater cannot
be dravoTi into
a Syringe
vohere there
is no ^ir í(J^
fupport it.

the

VVeightofthé
■^ir does not
at all raife
the Water ift
a Syphon.
Tab.lL
Fig. I.

r4. Vyhat is
the Caufe sf
the Vi^aters
rijlng in ths
Syphon»

I. You may find che Deícription
of an Inftrurhenc not much unlike
this in the Experiments of the Aca-
dtmy del Cimento, Btit che Air Pcimp

of the famous Mr. Hoyle exceeds
them all, and is fo well kriown, that
I need not delcribe it.

each

76

RO H JULI"’ s System Part I.

each of thefe two Arms is diminiüiedj Jn proportion to
the Weighc of the Water which each of theni itnpels j ,
and the Weight of the Water in the longer Arm being
heavier, than that in the Ihorter Arm ; it followSj tfiat
there remains more Forcé in the Air which a£ts upon the
Water in the VeíTel, to make it rife up in thefhorter Arm,
than there does in the other to repel ^ fo thaf it is in-
deed made to rife, and forced to rnn out through the
longer Arm, notwithftanding the Refiítance of the Air
which oppofes it.

yj. sríiíB%& 55-1 fuppofe, that the Arms of the Syphon do
the Armi of not exceed that Height of the Liquor which the Air would
mufi^h^for ^ perpendicular Tube j for if they be longer, the

'thiwatZto Liquor with which the Syphon is filled, will divide at the
afeend. Top, and defeend in each of the Arms ^ which is con-
firmed by Experience.

ys. ífcw the 5 ( 5 . Afcer fo many different Explications as have been
Air is draan gJready givcH, I don’t think it neceíTary to inlarge mu^
BeiC/sT’’ explaining how the Air enters, and is received inro

a Pair of Bellows j for it is eaíy to apprehend, that when
the Sidés are feparated from each other, they thruñ tor-
ward the Air, which not being able to move freely eve-
xy way i becauíé the World is full j or a^eaft not be--
ing able to enter in at the Noíe with Ear^, and quick
enough to fill readily that Space whicl^k^ft by the Sides
ofthe Bellows when they are opene* it is turned back,
and enters with Eafe and Swifcnefs through the Hoies of
the Bellows.

57. It is proper here to obferve, that we receive in the
Air by Refpiration, much afcer che lame manuer : For it
is certain, That the Mufcle of the Thorax and Abdo-
men, ferve to diftetid, and fwell the Body, by which
Means the Air being chruft back, gees into the Hollow of
the Lungs through the IÍ^.th and Noftrils.
yS. FTíence 58. The only Difficultpílj^ is, that lince v/e fuftain a
M ‘nfoTf Columns of Ai^li^rich are all heavy, and

fetsUyin which prefs upon the- external^^^pf our Body, and
in-eathin¿. thruíl it iiiwards j it íhould feem ought to feel-

fome Difficulty in breathing, in order^T%5.3t.come this

3 ’ 7 . How we
draxo in the
by Ref-
Diration-

Reíiftance : But the Anfwer is eafy j For if th

i't^>j^/ome

T. Bscaufe the JVorld is ful!)
Wbether the World be tull or not,
his thefame Thing i for it cannot
be, but that che Air by its own
Weight (and S^rin¿') muft ruíh into
The einpty BeUowfi when they are

open. Which I remarle
fiiow, that whacever becomes of tn^
Fülnefs of che World, che Explica-
ción of thefe and fuch like Mocions,
is che fame.

lo

17

Chap. 12. o/ N A TURÁL PkitOSOPHY.

to thruft it invs^ards, thére are alfo a fufficient Quanticy of
oiJiers, which encer into the Cavity of the Breaffc to
prefs it outwards ; fo that there is an ¡sq^uiíibrium between
thefe Forcea or Powers ; and this is the Reafon why we
ought not to find any Difficulty in Breathing, or if we do,
it is owing to fome other Cauíé.

59. The fucking in of Air through a Quill is done in
the fame manner as Refpiration j for it is the fame as if our
Mouth were as long as the Quill.

60. If we try to fuck a heavy Liquor through a Quill
dipped into it, we ought to find fo much the greater Dif-
ficulty as theQuantity of Liquor we make to rife is greater j
becaufe this Liquor preffing by its Weight upon the ex-
ternal Air which endeavours to raife it in the Quill, ^ hin-
ders it from impelling and affiíling the Air which is in the
Lungs, fo much as it ufually does ^ by which meaos the
Air in the Lungs is weakned, and has juft fo much lefs
Ifcrce to thruft the Parts of the Body outwards, than the
Air which is applied to the external Surface of the Body
has to thruft theto inwards, as the Liquo'r which is caufed
to rife in the Quill is heavier.

()i. I íhalL finifli what I have to fay concerning this
Sort of Mo®ns, with explaining that Swelling which
Surgeons max^g the Flefti, by the Application of Cup-
ping-GlaíTesj the'^tomon Methodof which, and that to
which all others may be reduced is this ^ They take a
fmall round Card, upon which they fix four Ihort pieces
of Wax-Candle, which they light, and fet like a Candle-
ftick upon the Part of the Body which they intend to cup:
Then they cover all the Candios wjfh the Cupping-Glaís,
but do not put it clofe to the Flem, ’till the Air that is
within it, is fufficiently heated j then as foon as it is

I. Hinders it from impelling)
ihe entire Weight of the Líquor^rV/
be confidered j we muft
Difficulty of Sucking, ^'^^¿refore
greater orlefs, becauíí-.'^^fóportion
10 che greater or Tbick--

nefs of ch^.J^^iñn of Liquor, the
Breaít^t^íülé^ or lefs diftended by
^ she of che Mufcles j fo that

Ith^pífliñance of che incernal Air
®tíí^vhich it endeavours to hinder
. che Liquor from rifing) /nuil be' ib
f much mote or lefs weakned by Ra-
refaftion, according to che Power of
the external Air, to raife up che hea-
vy Liquor to the Mouth. But be-
cayfe the Columns of Liquor are ral-

;rí''ed by tbe external Air with lefs or
greater Difficulty, according as they
are lefs or greater in Height and non
in Thicknefsi therefore ir we íuppofa
cwo íuch Columns, one of which is
twice as high as the other, and chis
other twice as chick ac that j chough
ic.be plain, that in both Cafes, there
is che lame Quanticy of Air to be
fucked ouc of che Quill, and che lame-
Quancíty of Liquor to be íucked
through it ; yec it is evídenc, that a
greater Diílencion of che lireaít, and
a greater Forcé of the Muleles is
required, that is, it is more difficult
to fuck or raUé up ihe Firíl than
the Second*

yp. Hoto it is
that we fuck
in Air^

60, JVl^y It is
more difficule
to fjick a
heavy Li*
qmr.

61. Concern-
ing the Ufe
of Cttpping-
Glaffes»

put

78

él. T%
the Ftcjh
fallís, ■■

ROHAULT^s SysTEsi Partí

put clofe, the Candles go out, and we fee the Flefli fwell,
and rife up.

(j2. In order to underífand the Reafon of this Experi-
ment, it is to be obferved ; that during that lliort time
that the Candles continué light, the Air which is in the
Cupping-Glafs, r choughvery much agitatedanddilatedby
the Flamej does however prefs upon the Fleíhj as much
asitdid before, becaufe the Cupping-Glafs being not yet
put quite clofe, does not take ofF any of the Weight,
which it had before ic was dilated ^ but it is otherwife af-
ter the Candles are extinguifhed by the immediate Appli-
cation of the Cupping-Glafs to the Body : Fot then the
Air which is contained in itj is no longer prefled upon by the
Air wichoutj and as itgrows cooler^ it has not Forcé fuf-
ficient to take up fuch a Compafsj as whcn it was agita-
ted by the Heat ; Wherefore lince all the other Parts of
the Body are preíTed upon by the external Air, which alfo
preíTes the Cupping-Glafs to the Body, the one muít^
neceflity enter into che other; that is, the Flefh muft k
thruíl: into the -CuppingrGlafs, and tljp Air within it
condenfed.

I, Though very much agitatcd and
dilated by the Flamci does hovoever
prefs upon the Flejh as much as it
did before, becaufe the Cupping-Glafs
being not yet put quite clofe, does
7 ¡ot take off any of the VFeight,
xohich it had before it was dilated*)

Thís Explicatíoi/ been fomc-
what more if ihe Aiithor

íáid — though JrJated by the Fíame,
yet jince much agitated,^ it

does howfver. Ñor was there any
?2eed of having recourfe to the Weight
of the external Air herc.

I, What is
meant hy the
Determinan-'
onof Motion,
2.. That fuch
Determinati-
on is fome-
thing difinCl
from Mption»
The firji
Froof

w

C H4,P. XIII.

Of the Deíermim^^ of Motion.

HEN a Body moves any pte&nlar way, the
Difpofition that it has to move tlrateiiay, rather

than any other, is what we cali its Determinatmi

2. Dsterminathn is a Mode which is dift inS^^di ec
from Motion, and which may remain the fame,
much foever the Motion be increafed or . diminiíh-t
ed ; Thus a Stone that falls freely in the Air, has a cer-
tain Quantity of Motion, and at the fame time has alfo
a certain Quantity of Bstermination of Motion dpwnwards;

and

79

Chap. 13. of Natural Philosoph y,

and if ic had been thrown oblique from the fame Place,
fo as co have come to the Ground in the fame time, it
would have had the fame Qiiantity of 'Determmation, but
a greater of Motion.

q. Another Proof that Determination differs from Mo- ■^.jinutúr
tion, is, that it depends upon a different Caufe from that
of Motion; thus in a Ball íf rucie by a Racket, the Moti-
on is owing to the Forcé vvith which the Racket is mo-
ved, but the Determination towards any Part is owing to
the Situation of the Racket.

4. Since every Thing endeavours as rouch as it can to ^-Thata'Ro-
continué in the State in which it once is, it is evident,

that a Body which has once begun to move with a cer- ugo ontofthe
tain Determination, oughtalways tokeep the fame ; that is,

. , , ° . h • 1 T • r 1 • • 1 ío move on m

it ought always to move in a Itreight Line; for this ts the ^ firáght
only Determination that is • natural to a Body in Motion : a>»í-
Wherefore when it was faid above, that when any Body
was moved in a ftreight Line, other Bodies muft neceffa-
*y be moved with a circular Motion, we are not to
think, that thofe which thus turn out of a ftreight Line,
tend to do fo of themfelves, but that they are forced to
do fo, by meetiñg with, and being impelled by other
Bodies. %

5. Thereh^^e when we fee a Body move in the Sides j-, rhat evt-
of a Square, tV-e (-TMcIude, that in the Places where it >7 ^"' 0 '
changes its Determnation, it is forced to turn out of the

way, by meeting other Bodies, the Refiftance of which, ccdtodofi.
it could not overeóme. So likewife if a Body moves
through the Sides of an Oótagon, we can’t but fay, that

I» Natnr¿il to a ’Bnáy in Motion)
Mr. Perrautt in his Tentam, Phyf
Tom. i. p. 80, 88. contends, {hac
Motion in a Gírele is as natural asín
a ftreight Line i for terreftrial Bo.=-.
dies turned round, endeavour loJj l
off írom the Center of their Mfr^Vn,
becaufe they areheavy ; but^^-^Üody
that had no Weíghc at^-'^Jíre turn-
ed round, ic would^.'^^We abouc its
Center freély /'^'í^^STany Impulíé,
and ■woul<L7':^^"'*endeavour to go oft
fromi¿'^^>Vüs if a Ball of Wax be
ib hollowj as to equal in

an equal Bulk of Water, it
comply with the Motion of
the Water' turued round in a Veflél
full of Water, that ic will always
deícribe the fame Gírele, and nevér
attémpc to go off from the Center
of iis Motion. But (befidcsthat there
is no fuch Thing as a Body void oí

all heavinefs) this AíTercion-is con»
trary to all Reaíbn, and this very
Éxperimenc proves nothingleísa chan
^acchis emiiient Perfon imagined :
.Jr whac can bé more evident, than
that this Ball endeavours to go off
from che Center of its Motion, buc
cannoc get off, becaufe all the Parts
of che Water endeavour at the fame
time to gooff from the fame Center,
and with che fame Forcé, becaufe
«equally folid 5 and therefore Cnce the
Sides of the VeíTel hinder them from ^
güing all ü’ff logecher, there is no'
reaíbn why the Ball of Wax fliould
recede from the Center, and impel
the Partsof the Water to che Cenier,
any more chan there isfor ihe Pnrts
of the V 7 acer to recede from the
fame Centerj and drive the Eall
thither,

ic

So

6. If thaí
Forcé ceafcs,
then it on¿hí
to move in the
Tangcnt of
thíjt Circle
tohich it >¿6“
fcribcd befare,
Tab. II.
Fig. 2 .

’ 7 . Bodies

«hich move in
a Circle, en~
deavoiir to go
off from the
Center of the
Circle vohich
they dcfcribe,
and make o-
ther Hodies
afproach (0 it.

8 . That a
"Bcdy in Mq‘
tion meethig
with amther
*Body xthich it
cannot mo^oC)
onght to be
r^eéled»

RO H AU LT^ s System Partí,

it is éight times forced to turn out of the way j and fince
a Circle is equal to a Figure of an infinite Number of
Sides ; it follows, that a Body which moyes in a Circle
is forced to turn out of the way every Moment, either
by the continual Refiftance ofBodiés which it every wheré
meets with, or~becaufe it is retained by fomething which
obliges it to keep always at the fame Diftance, and to
run through the Circle deferibed ,• otherwiíé it is certain it
would not deferibe a Curve Line at all.

6. For Example ; if the Body A deferibes by its Mo-
tion part of the Circle BCD, it muft be continually turn-
ed out of its Courfe from one of the forementioned Cau-
íes : Ifj when it comes to the Point D, it fliould be no
longer forced,' either becaufe the Bodies which it meets
with, fliould malee no further Refiftance, or the Thread
which connected it with the Center, and hindred it from
flying off, lliould break j it would not continué to de-
feribe the Ar'ch DEB, but it would deferibe a ftreighí
Line, which would run the moft direcftly that is poffiSe
from the Arch CD, that is, it would deferibe the Line DF,
which is the Tangent of this Circle, and malees the leaft
Angle that can be with the Circumference,and which, as you
fee, grows more and more diftant from Ahe Center :
This is confirmed by an infinite Numtf r of Experi-
ments.

7 . And lince a Body in Motion, ras always a Tenden-

cy to deferibe that Line, which it would deferibe if it
were at liberty j and what was faid of the Body A, is to
be undarftoüd in general of all other Bodies j we muft
conclude, that Bodies w'hich move in a Circle, have a
perpetual Tendeney-to recede from the Center of their
Motion ; and this they ought to do with a Forcé fo
much the greater, as their Motion is quick. Wherefore,
if the greater part of theí^ce contained in the Circum-
ference BCDE be full of BííSes which move round the
Center G, they will pulE all tlStófher Bodies with which
they are encompafled, and drive theü^ far from the Center
as they can : But if thefe Latter can Place to retire

to, they will be forced, in orderto give Plac¿^.,the other,
to go nearer the Center ; in the famemannerSí'ív^en we
dip our Handinto a Pail of Water, the Water is foK^^toi
give way to our Hand, and to remove from the BoftÜIEl
which it has a Tendency to by its own Weight.

8 . It is evident, that a Body lofes fo muchof its own
Motion as it communicates to other Bodies : Now if it
communicates no Motion at afl, to others, (we do not

here

Chap. 13 * o/ Natural Philosoi>hy. 8i

here coníider what may be occaíioned by its Softneís,

Weiffhc or FigureJ we have no Reafon to think tha: ic
ílaoufd acall abace of its Velociry. Whereforé if a Body
in Motion ñrikes upon another, which ic cannot move ac
aUj we ought to conclude, that ic wilí continué to move
on with the fame Celericy as ic did before ,• but bccaufe '

che Body which it cannot move, hindefs its Betelrmination-,
it muíl neceíTarily altee chis Hetermination, that is, it will
be refit^ied.

9 . This Determinación, may indeed be contra- 9.Thatther6
ry to che Firfi: but becaule the Nación we have of re-
fisEled Motion is noc difierenc from che Notion we have tn the Point
of direñ Motion, we ought noc to think that chefe Mo- "Z Rrfiexian.
tions are contrary to each other, but that » the one is on-
ly a Continuación of the other, and confequently, diat
there is noc any Moment of Reíl in the point of Reflex-
ión, as fome Philofophers have imagined.

10 ., Belides, if a Body which was in Motion, comes ’o-TkatRc-
•o be but one Moment at Reft, ic will have vvhollychang--f“¿“^^|'/^
ed its mannerofexiíling into the contrary, in which there </ there w«]-
will be as muela Reafon for its continuing, as if it had ‘• Mmentef
been at Reft a whole Age ; in che fame manner, as if a '

Body whiqt was. once fquare, was made round but one
Moment, ^will have as much Reafon as ever it had, to
• continué in Figure.

*ii.*When a'^wbdy falls perpendicular upon another, n- That a
which is hard and immoveable, it is evident, that the Re-
flexión ought to be made in the fame Line, in which dk«UriyHpoH
the Body moved before, there being no Reafon why ic ^

flioLild incline one way rather than another : Wherefore fifáld
there is na Difficulty in this Macter, except when the ímdUaiariy.
Line in which the Body begins to move makes oblique
Angles with the Superficies of the Body againíf which ic
ftrikes. But che Judgemen^e are to makc of chis, de-
pends upon what we are :o fay concerning che Cora-
pofition of Motion, wr^oí its Determinación.

I. The ene is of

the other] Buc^ j¿|jjjÍTOtío. For "Bo-
Mes rohich /’r^mPWjcr abfolntely hardi
río be void of Eiijfiicity-,
roílí^áfcrebotind from each other, 2m-
wability only niahes them ffop,
Optic.pag, 373. Sce above,
^Chap. X. Arde» 13.

Furcher, chere may be a Moment
of Ueft, in the Point of Refi exion :
becaule cherefíeóled Modon, is not

a Cominuation of che DireSfj buc á
new Motion imprefled by a new
Forcea viz.. the Forcé of EUificity,

As to what our Author fays i that
if che Body refted but one Moment,
ic ought- as much to continué in that
new ¡átate of Reít, as if ic had reíitd
a whuie Age 5 ic is Indeed truc, wícLi
regard to ihe former Motion i buC
fince ElajFcity is theCaufeof aneW
Motion, the Reaíou is very differenc.

G

C H A P.

%2 ROH AUL'T*¡ SYiTtu Partí.

CHAP. XIV.

0/ the Compfition of Motlon, and of iís Deter-
mination.

I. WhM

meant by com-
ponnd Mo-
tion,

Tab. II,
Fig. 3.

Tab. II.
ríg. i-

i! A L L .Motion that depends upon cwo or more Caufesj
n- ÍX cali Compound Motion : Thusj if one Forcé aíl-
ing upon the Body A, would caufe it to tnove along the
Line ABj and at the fame time another Forcé ading up-
on. the íáme Body A, would caufe it to move along the
Line AC, the Motion whichwill arife from the Adion of
thefe two Forcea, or from thefe two Caufes, will be 3
compound Motion.

a. Tm Qther 2. In order to find out whai Line the Motion, which
Mntions be- depends thus upon two Caufes, ought to be made inf*
find 7 h 7 am- '^t the two Liiics be drawn, which the Body wopld move
pundMotion.'m, if each of thefe Caufes produced their.Effed fepa-
rately. For Example, if the firíl: Caufe would in a given
Time, make the Body A move from its Pl^h, as far as
B ,• and if the Second Caufe would in thXfame Time,
make it move to C 3 let the Lines A^ be drawn ;
then having divided che Time in whim this Motlbn Was
made, into as many equal Parts as you will, divide the
Line AB into as many, by the Points E, F, G, and the
Line AC into as many alfo, by the Points H, I, L ; fo
that, if the firft Caufe aded alone, the Body A would
come to the Point E, in the firft Part of the Time ,• to
the Point F, in the fecond Part,- to the Point G in the
third Part ; and to the Point B in the Fourth ; and if the
fecond Caufe, produced l^^ffed íéparately, the Body
A would cometo the Pointnkm the firft Part ofTime;
to the Point I in the Second^Vto the Point L in the
Third 3 and to the Point C in the Fcl^j¿K After this, draw
the right Lines EM, FN, GO, BD, potÍí||^o the Line
AC,- and the Lines HP,IQ, LR, CD, paraWfeo the Line
AB ; This being done, the Points S, T, '-where

theíé Lines interíéd each other, will determine theíl^gne
in which the Compound Motion is made.

3. For itis certain, that the firft Caufe is anfwered, by
«f lempomid ®^^owing the Body to move to the Line EM in the firft
Molim. Part ofTime, and the Second is anfwered, if we allowit
to be found in the Line HP in the fame time ,- where-
fore both thefe Caufes are anfwered at once, if the Body

comes

5. \A De-
njonjiratim

Chap. i4« <?/ N ATURAL Philosophy. §3

comes to both the Lines EMj HP, at the fame Time ;
which it cannot do, but at the common Point S. Again, it is
evident, that the firíl Caufe is anfwered, if we allow the
Body to come to the Line FN in the fecond Part of
Time j and the fecond Caufe is anfwered, if it be allow-
ed to come to the Line iQ^in the fame Time ; and con-
fequently it is certain, that, in order to anfwer both thefe
Caufes together, it rnufl: be found in thefe two Lines ac
the fame time ; viz. in the Point T where they interfed;
each other. So alfo we may prove, that the Body ought
to be found in the Point V, where the Lines, GO, LR,
interfed each other, to anfwer the fame two Caufes ,• and
at laft in the Point D, where the Lides BD and CD in-
terfed one another

4. Where » the fimple Motions are equal, as in the firíi
Figure, the compound Motion is in a ¡Ireight Line : But

where the fimple Motions are unequal, as in the Second mt^ be mait.

^Figure, the Motion will be made 3 in a Line differently
curved, according to the different Inequalities of the ñm-
ple Motions.

5. If more than two Caufes concur to produce a coro- s- Howí»
pound iV|btion, it may be determined in this manner :

Fird drai^he Line in which the Body ought to be mo- ponndcd 0/
ved, fo as\i anfwer two Caufes; then, taking the Mo-""Y*"""*
tion in this Lintijas if it aróle from one Caufe only, draw ‘
the Line which it ought to defcribe, fo as to anfwer this

1. Sucb a Hnd of Motion as chis,
is that of an Arrow, in the famous
Experimenc of a Ship under full
Salí j where an Arrow being flioc
perpendicular, falls downagain upon
the fame Place on che Deck, whence
ic was íhoc : For che Arrow has a
double Motion ímprelTed upon ic aL

uuuüic iViULiuiJ luipjciica upoii il aL^tvii,uiuiJ!> wiiiuu aicucic

the fame time j one by che Bo^í^■ ^ ^compared with each o- Fig. 3

Hand which íhoocs ic, and tb*;, n-
ther by the Ship movinir; along.
Something like this was^viervM ac
Florence^ where a Lggjlra Ball íhoc
perpendicuIarl^^glIÍTOC ot a Muskec
Hxed in a AKgfíRn Carriage made to
^ifcly, fell abouc íeven
is fide the Mouth of the
, which moved Sixcy foiir Pa-
See Exper. ^cad. del Cimento.

145'. Perhaps the Muskec, wasnoc
erefled exaStly perpendicular, or
■was moved fomewhac fwifcer afcer
the Ball was íhoc ouc, than when ic
was íhoc í or if neicher of theíe
happencd, yej che ReGñance of che

G:

Air, which could not buc recard
the Motion of the Ball, might per-
haps be the fole Caufe why che Ball
fell fo much on chis fide the Mus-
ket.

2. The Jimple Motions are equal)
Ic isto be obferved, that chofe fimple
Motions which arehere

cher, and are called equal
or unequal, are noc chofe of differenc
Deterrainacions (fuch as AB , AC,) buc
che Parts of che -Motion of one and
the fame Determinación (viz. AE»
EF, &c. Afi, HI, &C.'} compared
togecher,

3. Tn a Line differently curved)
When one orboth che fimple Mo»
cions is alcered gr^ually and every
Momenc ; the Line which is defcii-
bed, may be conceived to be bent in-
co an infinite Number of fmall Lines
which end in a Curve. Such is che
Motion of projeífed 'Bodies. See the
izotes on Part eh, 3.2. 16.

Caufe,

ROHAULT^s SvsTEM Part L

6 . That the
Moúon of a
*Zall OHt of a
Cannon-i is a
coTnpotmd
Motion,

Caufe, and á Thirdj and fo on, if there be a Fourth or
fifth Caufej producing its particular Effed:.

6. Ic is eafy to íee, thát the Ball of a Cannon which
íéenis to be driven by the Fire level with the Horizon,
does notwithftanding move in a Curve like that defcribed
in the fecond Figure ; for there are two Caufes which con-
cur towards its Motion, the firíl of which, wz,. that
which caufes the Ball to move upon the Level, ought
continually to diminiíh, becaufeic communicates, bylittle
and little, its Motion to the Air which it difplaces : and
the fecond ought to increafe, becaüfe we find by Experi-
ence, that the Fall of a heavy Body is flower at the Be-
ginning than afterwards.

y. TheExadnefsof theCannoneerin levelling theCan-
‘thfMarkf Mark which he looks at^, ought not to make

íhoms that the US alter our Opinión, and to think immediately that the
3<í//áe/ce«¿í, Ball is carried in a ñreight Line : For if we obferve, that
the Cannon is not every where of an equal Thicknefs^^j
and that the Line AB by which the Mark is aimed ati
is at firñ above, but goes afterwards below the Line of
Direction CD^ we íhall conclude, that if the Ball hits
the Mark, ic has doubtlefs fallen a little, or dje it would
have gone a little above it. f

8 . ' As there are Compomd Motions, fo alio fk there com-

. That the

Tab. 11.
Fig.4.

8 . jVhatis

meant by com^

fotmd olTer- potmd Beterminations', and, ic may be, H hen the Motions

lab. II.
Fig- r-

9 . That ene
and the fame

are the moft limpie that can be : Thus we fay, a Deter-
mhiation is compounded of two others, when a Body
moving in a fimple Line to a certain Place, is at the fame
time carried two difFerent Waysj as if the Body A be
moved wich a limpie Motion from A tp B ; becaufe at
the lame time, ic continually approaches the Lines BC,
BD, we láy, that the Determimtion¡ by which it is carri-
ed from A to B, is comprended of two others, one of
which would make it go TÍ^ards D, and the other
at the fame time carry it froiriNj^^^to C ,• and thefe Di-
ílances are the Meafure of its Pro^jcE towards thefe dif-
ferent Parts.

9 . For the fame Reafon that we conM

'a-, ny one Be-

Deúmmatl- termmation as compounded of two limpie Bet’é^yeitmis,
en may be wc may as wcll confider it as compounded of innumt'^ble
m^nfdiffe- oc^ers. Thus the Bitermination from A to B ma^
rent enes, confidered as compounded of the Beterminations from
to E, and from A to F,- becaufe when the Body A
moves from A to B, ic continually approaches BE and BF

alfo.

85

Chap. I4- «'/’ N ATURAL Philosophy.

alfo^ from which ic was diílant by che Length AE and
AF I.

I. From chis Principie, che Me-
thod of explaining che Forces of che
Mechanick Povoers (as they are cal-
ledj) may excellently» well be de-
duced.

For fiQce a Body with two Uni-
ted Forcesj ahvays dercribes che Dia-
gonal of a Parallelograma in che íáme
Time, as íc would do che Sides, if
che Forces were feparate j ic is evi-
denc, chacany Forcé whacfoever,a£t-
ing in a given pireftion, may be look-
ed upon as the^ Effeft of two other
Forces a£ting in Direéfions, which
ac the íame Point, íliall on each fide,
be any wayinclinedto the given Di-
refHon, providcd they make an An-
gla leís chan cwo righc ones : And
chis is abundancíy confirmed in Me-
chanicks s for by íuch a Refblution
I of a given Forcé inco two others,
theknown Propertiesof the Mecha-
nick Powersj fuch as the 'Pallance-,
the indintá Plams &c. may eafily be
deduced.

Of che "Balice or Leaver* Prop. I.

It cwo Forfl)^, which a£]: upon che
Arms ot a BaUlmce in given Dire-
¿fions thac are in^lfe fame Plain
with thoíe Arms, ballancc one ano-
ther,‘ chefe Forces are co each other
reciprocally, as Perpendiculars 1er
fall from the Cencer of the Ballance,
co cheir Diredtions.

DEM.— .ZVifwf. Princ. pag, 14.

Lee C be che Centerofthe Ballance.
Cpj CP che Arms,

Tab. XX. PA the Direclions of rhe-
Fíg. I. Forces afting upon

Arms Cp, CP. Lp aCí.
be drawn perpendicular to and
CD co PA, meeting thftjj^ m E and
D. On the Cence^fl^md with the
RadiusCE, vi^ 0 ^longQÍi of the
PerpendicuJi^^ec a Gírele be de-
ch fhdll incerfedt the Di-
the Forcé P in A, and let
iCAbedfawn t to which let
rbe drawn perpendicular, and Gt
parallel, meeting DPA in F.

Iris evidenc. thac che Arms of che
Ballance CP, Cp, may b? Jooked up-

as Lines thac will noc bend, ly-
^ng in tiie Plain moveable abouc the
Cencer C ; and che fame may be
underífood of any other Lines drawn

through the Center C, and lying in
the fame Plain. Nov/ fince it is ma-
nifeít, íhat there is no differencein
whac Points of che Lines, in which
che Forces P and p a£l:, thoíe Forces
are placed j Gnce whereíoever they
are in thofe Lines, they v/ill have
exaálly the fame Power to turn che
Plain CDApE abouc Ks Center: the
Forces P and p may be fuppofed to
bein the Points A and E. Then che
Forcé P, íuppoíed to be in A, may
be reíblved (as was before obferved)
into two other Forces : One of which
may aiSt according to the Line CA
produced, and the other, according
co the Line AGj and which may be
to each other as FG to G A j buc each
of chem íingly co P, as FG and AG
fingly coAF; as wiJI be evident, if
che Triangle AGF be compleated in
the Parallelogram AGF^. Ir is alio
manifeíf, that the Forcé, which is as
FG, and which a£ls according to the
Line CA palfing through the Center
ot the Plain, does notning at ali to-
wards turning that Plañe abouc the
CenterC; buc cheForce which is as
AG, and which draws the Line CA
perpendicularly i fince,by che Hypo-
thfñs, ic ballances cheForcep, which
draws the L’ne CE, equalco CA (hy
Conjlrtidiou) perpendicularly alio, ic
mnft neceá'arily be equal to ic.
Whereforep will be to P as AG co
AF s or as DC {by reafon of the fimi^
¡arTriangles\:Qh-i ACDj to CA or
CE ; Thatís, the Forces/» and P are
co one another reciprocally as Per-
pendiculars lee fall from the Cencer
to che Lines in which they

Coroll,

If the Armj lie in a ftreight Line
•and che Decerminations of che Forces
be parallel, ic js evidenCj thac the
Forces are reciprocally as che Length
of the Arms.

2. Henee alio, in che Angular Bal-
lance PC/», which curns
about the immoveable Tab. XX.,
CenterC,' cheSituacion Fig. 2.
which it will bein,when
any cwo given Bodies are fixed to
íhe Ends P and p, may be decermi-
ned. Foi if che Line P/> which joins
che Ends of che Balhance,- be divided
in reciprocal Proportion co the
Weiehcs, and the Point of Divifion
G 3 Tb?

86

,jo. That it
not necef-
fary to Qonfi-
der all the
Determina^
tions ofxchich
Otte may be
sompofed.

RO H JULIAS System

Part I.

lo. But it is not neceíTary to confider all the fimple
Determinatiom, of which One may be compofed ; It is fuf-

ficient

T be made in the Line CT drawn
through che Center, parallel to the
Direñion of che Weighcs ; I fay ic ís
done: For PD and being drawn
parallel, and DCE perpendicular to
CT i it is evident chat DCE is di-
vided in C, in che fame Proporción
that PTp is xn T, and that the
Weighcs may be fuppofed to be pla-
ced in che Poincs D and E, Whefe-
fore chis will be the Situación of
the Points P and p, that is, of the
Ballance it felf when che Weighcs are
in aquilibrio*

3. In the Ballance or Leaver, ic
is evidenc, chac cwo For-
Tab.XX. ces, fuch as P andp which
Fig. I . when the Ballance librares
to and fru, are reclpro-
cally as the Velocicies of che Points
D and E, reckoned according to the
Direftions of chofé Forcesa will bal-
lance each ocfaer.

Of che Incltned Tiaín> Prop, 11 .

Ifa Forcé, withagiven Direflion,
íúpporcs aWeighc upon an inclined
Plain; that Forcé isto the Weight,
as the Sine of the Inclinación of che
Plain, to the Sínedf che Anglewhich
is made by the Line in which the
Forcé a£ts, and che Line perpendicu-
lar to the Plain.

DEM-

Lee AB be the inclined Plain, P che
Weighc íupporceda DPV
Tab.XX. théDireftion of the Forca|
Fig. 3. which fupports ene Weighcj
Lee PC be drawn perpen-
dicular to AB 5 and trom the Poinc
Ci Jet CB be drawn parallel to che
Honzon, and perpendicular' ro the
coinmonSeftionof che Plain and che
Horizon, meeting the Plain in B 5
and CA perpendicular co che Horizon
and alfo co CB, meeting the Plain in
A, and the Line in which che Forcé
acts in V.

Now P may be conceivM te be
heldunmoved by ehree Forces aiSling
together: onepf which is the Forcé
of the Weighc ic felt tending down-
wards in a Line parallel ro VC 5 che
Second is che Forcé a£ling in che
Line DP V , and che Third is the Re-

fiftanceof the Plain itíélf, affing in
the Line CBl perpendicular to the
Plain : Buc chele chree Forces are co
each other {(rom vohat toas faid be^
fore) as the Sides of che Triangle
VPCi as wíli be evidenc, bydraw*
ing a Line through P parallel to VC,
and compleacing che Parallelogram*
The Forcé chereiore is co che Weight
which itíüftains, asTV C0VC5 thac
is, as che Sine of che Angle VCP, or
ABC, coche Sineof the Angle CPV
or CPD. ^ £. D,

Coroll,

1. Ifthe Poincs V and A coincide,
thac is, if the Forcé a6ls according
to che dire£lion B A, che Angle CPL^)
will be a right Angle j and cherefore
in that Café, the Forcé is ro the
Weighc. as che Sine of the Inclina-
ción of che Plain, co che Radius, or
as che Heighi of che Plain AC, co ics
Lengch AB. ' And inifiis Cafe, the
Forcé which is requjp-d’co íupporca
given Weighc is leat^cf all s becauí»
the Proporci^, of che Sine of che
Inclinación cffíhé Plain, to che Radi-
us 5 is leís than its Proporción to any
other Sine whatfoever.

2. If chePomt V falls above A;
the greater the Angle APV isj ib
muen che more Forcé is necefíary to
fupporc the given Weighc upon the
Plain AB. Iníbmuch, thac by in-
creafing the Angle APV, che Propor-
ción of che Sine of the Angle ABC,
to che Sine of the Angle CPD, is al-
io increafed, ’till PV, AV, becom-

^parallel, and the Angles VC?,
CPD forthac Reaíbn equal, the Forcé
and the Weighc will alíb become e-
qual.

3. SoIilSUi^^f the Poinc V falIs
below A, as aWSl^p Forcé requi-
íite co fupport the^v^^^Weighe, is
again increafed i the Angfe" '^^eing
increafed, cill P-y, v C become'Vfal»
and ihen ihe Forcé and che W?^
will become equal again. Furttf ^
when che Lines Fvy PC coincide, and"^
the Angle -y PC by thac meansvaniíhes,
the Sine of che Angle ABC will
bear an infinite Proporción to*^e
Sine ofthac 5 chat is, no finice ForS
whaiíoever, añing ín a Line per-
pendicular co the Plain, will be able

19

Chap. 14” Natural Philosophy.

ficient to confider thofe which we have occafion for in
the explaining any DiíBcukies ^ herein imitacing Geome-

tersj

íp rupporc the Weight upon the
Plain.

4. If the Line in which the Forcé
añs be parallel to the Safe of the
Plainj the Weight is to the Forcé
which fupports it, as BC to CAj or as
che Bafe of the Plain to the Height
of it.

¡, It from the Point P, PF be
let fall perpendicular to
Tab.XX. EC, and from the Point
rig'4. C, CG perpendicular to
VP; it will eafily ap-
pear, that PV is to VC (that is, the
Forcé is to the Weight) as CF to CG.
Wherefore the Forcé and che Weight
wdl then fupport one another upón
an inclined Plain, when they are to
each other recipr^ally as Perpendi-
► ¿ulars drawn from the Point C to the
Lines in which they aél; (or, if
GCF be looked upon as an angular
Ballance moveable about the Cencer
C) reciprocally as the Velocities of
the Points G and F reckoned upon
the Lines ii^^ich che Forces aél.

Of the fFe<(gí.^rop. 3.

If three Forces aíling together up-
on an Ifofceles W edge, in Lines per-
pendicular to the three Plains of
tbeWedgei two of which Forces,
víz. thofe aíling upon the Sides are
equal to each other, and the Direébion
of the Third which aSs upon the
Bafe of the W edge, pafles through its
Vértex ; if, I íáy, thefe three Forces
fupport each other, che Forcé aíling
upon the Bafe, will be ’to fhe otl^i
Two, as the Bafe ot the Wedge..^TO
fhe Sum of its Sides,

ED, ed. Let EF, e f be dríwn
parallel to AB, and DF, df, paral-
lel to GC, fo as to forra che Trian-
gles DEF, def. Now each of che
Forces ED, ed, raay be imagi-
ned to be refolved into two o-
ther Forces, which are to each other
as EF to FD, and e/to/d: And
to aíl in Chole Lines : And thofe
two, which are as EF, e/, becaufe
they are equal, and oppofite, will
deílroy each other. Bií the Forcé
which aíls upon the Balé AB, in the
Line GC ; becaufe it fupports the
two other Forces FD, / d : boih
which are che lame way, and a£l in
a contrary Direílion to Chat Forcé
upon the Bafe ; is therefore equal to
che Sura of them. The Forcé there-
fore aíling upon the Bafe of the
Wedge, is to che Sura of the Forces
aíling upon its Sides as DF df
to DE -(-de or {hy the fimilar Tri'^
angles) AG +- GB that is AB tp
AC “í— CB.

Coroü,

íeprefent a Wedge ; and
lee CG be perpendicular
to AB, and GD, Gd
perpendicular to AC,
BC,' and thefe will be
the Dlreílions of the three Forces.
In the Lines GD, Gd produced, let
PE and de be taken equal to each
bther, which may therefore repre-
fent the two equal Forces; which aíl
upon the Sides, in the Dirfílions

The Velocities of the Wedge, and
of the Body relilling it, reckoned iü
the perpendicular Direílion before
explained, are to each other recipro-
cally as the Forcé aíHng upon che
Bale, to the Forcé aíling upon che
Sides of the Wedge, when theíé For-
ces are in tequilibrio.

For when the Wedge ABC is dri-
ven up to che Top, or is
in the Situación a he, it Tab.XX,
is erident, that the Parts Fig. 6 '.
of theBody that is cleaved,
have receded from each other, the
Length^d or GD, in the Direílion
of the Line perpendicular to AC
ot a a GC therefore is the Veloci-
ty of che Wedge, and Gd che Ve-
locity of the relilling Body. Btit (íji
the fimilar Triartgles) GC is to GD,
as AC to AG, that is, as AC -t- CB
to AB . And che Proportion will be
evidencly the fame, whatever Situ-
ación the Wedge be in, between the
Parts of che Body to be cleaved
by it.

e 4

oí

88 ROHAUL^’

tets, who do not draw from

Of the Screx». '

‘ A Deñnition»

If the Plain of the Triangle ABC
(whofe Hypüíhenufe re-
Tab. XX. prefents fuch an inclined
Fig. 7. PlaÍD, as was explained
ábove in the ad Propofi-
TJon) be conceived to be fo fitted co
the Concave Superficies of a hollow
Cylinder (the Circumference cf whofe
Bafe is equal to the Line BC) that,
the Plañe ABC cotnciding with the
Superficies of the Cyiinder, rheLiae
BO may be bent into the Periphery
of a Circle equal and parallel co the
Circumference of the Baíe j che Line
ÍÍ 3 A sWill form a kind of Spiral, af-
cending upon che Cylindrical Super-
ficies, and íurroundlng íc once: So
likewife, if feveral Planes> íuch as
A a Ci equal and fimilar co the for-
'rner, and whoíé rlghc Angles are
fubtended by che Line BA produced,
be imagined to be fitted in che fame
manner, coche fame Superficies) di-
ílant from each ocher, by che Space
AC OT a c (their common Heighc)
there willbe many Spirals formad by
•the Lines A &c. all continued
from one có another) and each of
chem once furrounding thé Cylindrí-
cal Superficies. Furcheta if ocher
Planes fimilar and equal co ABC be
conceived- i n che fame manner tobe
ficced to che gibbous Superficies of
another Cylinder, whofii Bafe is e-
qual to the Baíe of che Concave Su-
perficies of che fnrmer Cylinder i
there will by chis means be Spiralá*
formed in chis gibbous Superheies,
exadlly like choíé in che Concave
one before. Now if the latter Cy-
linderj v^hich may be turned about
SvS AxiS) by means of a Leaverpaf-
fing through che Center of any of
ics Bafés, and lying ín che Plañe of
rha: Baíe» be imagined to be ío pla-
ced wiihin che former Cylinder)
whichis ñxed and immoveablc, chac
the Superficies agreeing, che Spirals
formed ib each Superficies, may a-
gree wiíh one another alfo ; and if íc
be ib contrived, chat chey íhall aU
y/ays chus agreej v/hen che incernal
Cylinder is turned abouc its Axis, and
■ks Bife recedes from orapproaches to
jheBaíéot che external Cylinder j it

s System Part 1.

one Point all the Lines thac
.can

is evident) thac cwo Screws, che
Male and the Female may be coUr
ceived to be chus generacedp

Prop. 4.

In the Screw, as the Alcitiide of
one Spiral) is to che Circumference
of the Circle, whofe Radias is che
Leaver by which che internal Cylin- '
der is turned round j fo is che Forcé
perpendicularly applied co the End
of chat Leaver, to che Weigbc lifced
up by the Screv/, when che Forcé
and che Weighc are in aqnilibrio*

DEM.

Lee the Axisof the Screw be per-
pendicular to the*fcori-
zonj and thePofition of Tab. XX.
the Leaver, by which che Fig. 8.
incernal Cylinder is
turned abouc íes Axisa will be Hori-
zontal. Letche Weighc beplaceda-
ny where in the Line^nf the Axis ;
and then chat Weigh^/’by means of
che incernal Cylindery ^iil prefs wich
equal Forcé (m Di^dlious perpen-
dicular to thfj^^Iorízon) upon every
individual Poinc of the Spirals of the
excernal Cylinder j aud che Sum of
the Forces with which all thofe
Points are preíTed, will be the fame
as the whole Weigbt to be lifced
up. Buc let US firít confider the
Forcé, or chat pare of the whole
Weighc, which preffes upon any one
particular Poinc. FIow ic is eaíy co
fee, thac the fame Forcé, in a hori-
zontal Direñion, which is able to
jgipporc the Weighc, which preíTes
upen any one Poinc of the Spiral,
upon the inclined Plain of which
thac Spiral is formed j thac fame
Forcé wicCf^fje íáme Diredtion, is
alío fiifficien'c,-'^', fupporc thac fame
Weight upon th^^t'^^iü i and that
there is plainly no diftL^jice, wher
ther chis Forcé be immeti.!!”%^ ap-
plied to the Poinc which is pre^d ;
or be in any ocher Line coucfiÍD^&«
Bafe of the incernal Cylinder. Lc^
BC therefore be the Circumference
of thac Bafe; AC the Radiuss AG
the Leaverby which cheinternal CyS
linder is turned abouc its Axis-i FGH
che Circledefcribed-by che Radíus AG.
Thefe Thingsbeing íuppoíed j from
whac has been laid, cogether with
the

Chap.' 14 - o/ NaI-ural Philosophy,

can be drawn from it, but fuch only as they think may
be of ufe in their Demonftrations.

the Definition of a Screw, and the
4th Cqí^IL of the 5 d Prop. ic fol-
lowsi Chat, ascbeHeighcofone Spi-
ral, to the Periphery BC j fo is the
Forcé apphed to che Point C, in a
pireñioQ perpendicular to AC, to
thac pare of the whole Weighc, which
thac Forcé fupporrs upon any one
poinc of the SpiraU And {by the
Froperty of the Leaver) as the Cir-
cumference BG, is to che Circumfe-
rence FH i (chatis, as AC to AG ,)
fo is che Forcé exercifed in G to che
Forcé exerciíéd In C s beeaufe che Di-
reftions ot chefe Forces being paral-
lei, they have equal Power on che
Leaver ACG, whofe Centér is A.
Therefore {eqnally by Fer turbación')
as che Height o£ one SpiraU to che
periphery FH j ío is che Forcé which
^ercifedin'G, íupports thac pare of
che whole Weighc, by which any
one Poinc of the Spiral is preíTed ,• to
Chat pare of che Weight ic felf : And
as the Forcé which fupports.that one
particular Pai^f che whole Weighc,
IS co thac ODcl»arcicalar part of che
Weighc; foisiSe Forcé which, act-
ing in che íáme ÍJireífion, fupporcs
all che Pares of the W^^^c, thac is,
the whole Weight; to^lthofe pares
together ; thac is, co fuppoic che whole
Weight. Therefore, &c. Q. ED.

Coroll,

The Circular Velocity of thac
Forcé by which che Screw is turned
round, and che Veloctiy of che
Weighc which is lifced up by means
oi: the íícrew, are to each ocher re-
ciprocally as thoíe Forces wben they
are in aqiiilibrio. por ic ís evidenc,
chai in a whole Revolución of che
Leaver, the Weight is raifed juft the
Height of one Spiral, and thac in
■every Pare of the Rev^ fon, c
Weighc is raifed pmp^lKonably.

Of the Pnl^i or Windlefs, Prop. y.

fs evidenc, thac che Pulley may
Kcounted for, in che fame man-

ner as the Ballance or Leaver, i ti
which the Forces are imployed ei-
cher on the fame Side of the Cencer,
oronbochSídes: Which, whenchey
are in aqtdlibrio, are to each ocher re-
clprocally as Perpendiculars lee fall
from the Poinc which reprefents che
Cencer of che Leaver, to cheir Dlre-
dtions. And henee the Forces of
Engines, which coníitt of many
Pulleys, according as they are diffe-
rently framed, may eanly be ex-
plained* If che Compoficion of che
Pullies, orchemanner offraming che
Windleís be íuch, thac che Ropes
which are ficced to the Pullies, are
parallel to one anocher ,* and the
Weighc be fo íufpended in che midft
of che Ropes, as to draw every one
of chem wich equal Forcé ; ic is íelf
evidenc, cha: the Forcé, is to che
Weighc which ic íupports ; as One,
co the Number of Ropes. For when
thac Forcé is applied fo one of che
Ropes only, it is direftly oppoíed
co thac part ‘only of the whole
Weight, which draws thac Rope ;
che Pin co which che Windleís is
fixéd, fupporcing the ocher Parts of
the whole Weighc.

' Ic is alio evidenc, thac in chis En-
gine, tbe Forcé • and che Weighc,
when they are in aquilibrioi are co
each ocher reciprocan y, as cheir Ve-
locicies, when the Forcé raiíés che
Weighc. For ic is manifeít, thac
tbefe Velocicics are to each ocher,
as che Decreafe of che Length of all
the Ropes which ííipport che Weighc
taken together, co che Increafe of
che Lengih of the Rope co which
che Forcé is applied, in che fame
time; and thac juft ib much as is
loft in a given time xnall che Lengeh'^s
of the Ropes which fupporc che
Weighc; the very íáme is gained,
in che fame time, in che one Length
of thac Rope co which che Forcé is
applied.

CHAP,

9©

ROHAULr^s System

Pan I,

T, 7V/jaí it
mt^nt hy
JR^exim
étod Refra^

Qhn^

a. An In'
fianct of Re~
Jlexion,

Tab. II.

V¡s- 6 »

5 . That t^e
Angle i>fKe^
jiexíon is e-
'^ual to the
Avgh oj In-»
0 demt»

%

mmmmmmmmmmmmmmm

C H A P. XV.

0/ Reflexión and RefraSiion.

T H AT we mayapply what has been faid to lome Ad-
vantage, we Ihall, by the help of ic, expíalo the
Manner of Reflexión and Refraíiioa. But to avoid the
Error of the Antieats, who confounded thefetwoThings
together, we may obferve,* that by Reflexión is meant no-
thing elfe but the Bending, or Alteration of the Betermi-
naiion, when a Body in Motion, ftrikes againft another
Body which itcannot penétrate j máhy 'R^aSiionis msaxA
the Bending or Alteration of the Determination, when 3
Body in Motion^ paíTes out of one Médium into another,
which receives it with more or lefs Difficulty.

2. Suppofe, for Example, that the Body A, which ^
perfe£tly hard, moves with a. limpie Motion, in the Line
AB, and that it meets with the Body CDEF, which I
fuppofe to be perfedly hard likewife, andnot tobelhaken;
Then, from what has been faid, it follows, ^at the Body
A ' ought to continué in Motion, becauf|‘it does not
communicate any part of its Motion ; and it ought to be
ílruck back, becaufe it cannot go on^i a ílreight Line :
But let US fee how, and which way ; And that we may
not multiply Difficulties, we do not now coníider, what
will arife from its Bignefs, Figure or Gravity : Let us
fuppofe likewife, that the Air makes no Refiílance to it,
and that it moves with equal Velocity.

3. This being fuppofed, let a Circle be defcribed on the
Genter B, and with the Diftance BA ; and for the fame
Reafon that the Body A comes from the Circumference
to the Genter in a given Time, it ought to go from the
fame Genter to lome Point of the Gircumference of this
Gircle in the fame Time ; Now to determine that particu^
lar Point ^ from the Points A and B^'íít the Lines AG,
BH be drawn perpendicular to the SupefÍMes GF ; and
the Line AHI, parallel to that Superficies: N(í&:"-we may
obferve, that though the Body A is carried with a^^^pla
Motion, it is however true, that with reípeót to the^fe I
dy GDEF, its Determination in the Line AB, is com-^
pounded of two others, the one of which makes it go
towards the right Hand, by the Length of the Line AH,

03 ?

i Ca¿ht iDcontíim in Mftíon) See above, Chap. x. Art. 13.

I Chap. 14« c/Natural Philosophy.

or which ís equal to it, GB j and the ocher makes it come
downwards towards GB, by the Lengch of rhe Line AG.
Now we raay further obferve, i that the Body CDEF
refiíls the Determinación downwards, but that it does not
ac all refift the Dctermination towards the right Hand,
that is, that part of the Motion which is determinad to-
wards the right Hand, which confeqnently ^ ought to
continué as it began. So that the Body A having in a gi-
ren Time with chis Determinación, pa'íTed ihrough the
Space contained between the Lines AG, HB, chatis, mo-
ved che Length ofthe Line AH or GB, it ought in the
fame time to país through an equal Quantity agaín, or
which amounts to the fame Thing, it ought at the End
of chis Time, to be found in the Line IL, which I fup-
pofe to be perpendicular to the Superficies CF, and the
fame' Diftance from HB, as HB is from AG. So that,
to fatisfy that part of che Motion which is towards the
5 which does not alter at ali, we find that the Body A

í ¡n the giren Time, ought to be fomewhere in che
6 Line ÍL. But to farisfie the whole Motion, we haré
I before fhown, that it ought in the fame Momenc to be
! fomewhereLin the Circumferenae of the Circlc : l’herc-
I fore, that %eíé two may be both fatisfied together, we
ought to conclude, that it will at the Lme Time, be in
the Circumferenc^of the Circle, and in the Line IL to-
gether, which can be no where elfe but in the Poinc I
which is common to them both. Thus we fee the Body
A which began to more in the Line AB, is reflefl-ed in
the Line BI, which makes with the Superficies CF the Anglé

I I. That the *Body CDEF refijís the
I Veterminatim) If che íncidenc Body
I A, and the Body CDEF upon which
I it fírikesj are void of all Elaíiicicy 5
ihe Body CDEF not onlyreíilis chis
perpendicular Determination? but en
tirely deñroys all che Mocion that
arifes írom that Determinación (See
the Notes on Chap, ce. Art, 13.) fo
Chat the Body A, is .«{tcrwards mo-
ved, with the otbei* pare of ics Mo-
cion only, along che SuperficiesBLF.
Buc if eicliwT, or both thefe Bodies
be n'juteñly elaílick, then a new
íi^-.on will be xmpreíTed upon the
¿üdyA, equal co the Mocion which
was loft, and with a contrary De-
terminación i ío that, when it comes
to che Superficies GL, with the De-
terminación AG, it will then recede
from it with the Determinación LI.

This Is carefully to be obferved, be«
caufe ic is necefláry co che com-
pleating this DemoníiratKm, Yy
which ic appeárs, chac che Angles of
Incídence and Reflexión are equal,
For che Nacure of this E¡afi¡i.k Forcé
being rlghcly underflond, the De-,
monftration concerning the refledl»
ing of Elaftick Bodies, . wili hold ir;
che fame manner as in perfeflly
hard Bodies, according to the /\.u-
thor’s Principies, hee lurther, }he
Notes on Chap, Art, 6. Tab, II*
Fig. 6.

í. Otight to continué as It hegan)
Henee ic foUows, chac che Lines of
Incidence and Repercuflion are in a
Plañe perpendicular to the Superfi-
cies of the refle£ling Body. See the
Notes on Chap, xxxiv, Art, 2»

ibl;

9 *

Tab. ir.
Iig.6.

<52

4» -An £.v-
ample of one
Sort of Re-
fraélíon.

Tab. líl.
Fjg. I-

S.Another
Sort of Re-
fraÜion,

Tab. III.

Fig- I.

6. Whtn a
'Bodj is turrf
ed Qiit oj its
Courfc, roe
fnnft thinki
that it meen
mith fome
Objlacle m
that part
from íohich it
tttrtit*

Tab. II.
Fig. I.

RO H AULT’ s System , Part I,

IBL, which is called.the Aagle of Refleñion, i which
may eaíily be demonftrated to be equal co che Angle ABG,
which is called che Angle of hicidence.

4. Let US iiüw come to Refraíiion ,• and that we may ex-
plain the Nature of it fully^ I íhall h'ere make ufe of the
Example of a-Ball, as was befóte done in Reflexión.
Suppofe thcn the Ball A to be moved along the Line AB
in che Air, but ftriking obliquely upon the Water below
CD, inftead of going on diredly towards E, it tends to-
wards F ; this Sort of bending, a meafured by che Angle
EBF is what we cali Refrañion.^

5. If the Body A^. afterit is'arrived at B in the Line
ABj inftead of being turned towards F, is turned to-
wards G j chis is RefraHion alfoj but of a difFerent Sort
from the other : Nowin order to diftinguifti thefe two
Sorts of RfraSiion, let che Line HB, be drawn thfough
the Point B, where the Body A paíTes out of one Mé-
dium inco the other, perpendicular to the Superficies CD,
which divides the two Médiums, and the Kind of RefrB
Uion is detennined, by the Approach to, or Recefs from
this Perpendicular. For Example, if the Body which
moves along the Line AB, when it is turned^^ out of the
way, afcerwards moves along the Line BF, |*íni»is called
Refra&ion from the Verpendicidar ,• but if it afterwards
moves along che Line BG, then it i|^, called 'RefraBion to
the perpendicular.

6 . Thefe two Sorts of Refrañion ; bave been obferved a
long time, but the Caufe of chem was noc ac all known.
And we may venture to fay, that this is one of thofe
Things which the Antienes were ignorant of, and the
Difeovery of which is owing to one of the principal Men
of this Age; and agreeable to his Opinión, I thus explain
this Matter : Since we are fure, that every Thing, as
muchasiccan, perfifts in that State in which it is,- after
we find by Experience, that a Body quits the ftreight
Line in which it began to move, we muft neceíTarily
think, that it has has met wich fome Obftacle on that pare
from v/hich it removes : Thus, if, when che Body A is
come to che Point B, it is turned out of its Courfe to-
wards the Point F, we oughc to conclude, that it\íneets

1. Which viay eajily he dentmjlta-
ted) For BL =: GB by
Tab. ÍI. the Hypotheíís j and
Fig. 6. LI = GA, bícaiife GL
and AI are pirallelj and
Angles L and G are righc An-’

glesj by theHyp. ThereforetheTri’
angles ILB, AGB are equal and fi-
mijar.

2. Meafured by the Angle EBF)
See the Notes mon Art» 11. of this
Chop.

93

Chap. 15. of Natüral Philosophy,

wich more Reíiftance on the Side M, tban on the Side
Ni and if ir is turned toward G, we have Reafon to
thinlc, on the contrary, that ic has mee wich more
Reíiftance on the Side N, than on the Side M.

7. We may realbn in the fame mannerj in order to
determine on which Side, a Body moving out of one
Médium into another, will be turried. , For íince we be-
fore knew, that the unequal Reíiftance, which a Body
in Motion meets with on diíFerent Sides, (according to
the diíFerent Médiums through which it paíTes) would
forcé the Body to turn out of its Couríé, and to remove
from that Side where ic finds che moft Reíiftance •, when
once we come to know, that there is more Reíiftance on
the one Fide than the other ; we conclude, that ic will
turn out of the way, by removing from the Médium
where che Reíiftance is greateft. And thus when we once
come to know that Water reíifts the Motion of a Ball
more than Air, we ought to think, that the Ball which
moves in the fe.ir from A to B, in paffing into the Wa-
ter which is below B, will turn towards F, and fo will
recede _/rí)«2 the Ferpendicular.

8. This may be applied « to all forts of Bodies, and to
all forts oí^lediums:, and therefore we may lay ic down
for a generaf Maxim, that when a Body paíTes obliquely
out of one Mediu^inío another, which malees a greater
Refiftance to it j í^ughe fo to turn as to remove from the

! ferpendicular j and, on the contrary, when it paíTes out
of one Médium into another, where ic finds lefs Refift-
ance^ it ought to be fo turned, as to approach'Zewíjríiíf
the Perpendicular.

5». I exprefsly added, that the Body which paffes out
of one Médium into another, muft fall obliquely upon the
Superficies which feparájtes the two Médiums, in order to
be refradedj forif it falls perpendicularly upon this Super-
ficies ; as there is noching to refift its Motion more on the
one fide than on the other, fo ic ought not to be turned
out of its courfe at all, ^ but to continué tú move in the
fame Line.

10. The

yfVo all Surtí ofUodieí) For this
Píiíaíon the Rays of Light whích
país oucof Airinco Water., arerefleñ-
ed íDvoards the perpendicular, contra-
ly to what we fee in a 'Bali tbrown
out of oLir Hand ; becauíe Water
whicKrefiñs the Motion of the Ball
more than Air, on the contrary,’ re-
fiíís -Light leís. {See Cha^, zy,

158.) or to fpeak more tnily, it acce-
lerates the Motion of Light more by
atiraftíng it 5 as wiJl be íhown af-
cerwards.

2. But to continué to move in the
fume Line) Yetfomehavethought, as
J, FoJfms,Willebrord Snelli that chey
have leen a perpendicular Ray of
Light, fonje way refradting and con-
tra¿tied

7. That the
Body in Mo^
tion, recedes
from the Me~
diíim which
makeí the •
greatef Re*-
fifiance to ih

8. TheJVay
hoto to aeter^
mine the
ticular fort of
Refraéiion,

9. That a
Body rokich
falis pnpen-
diciilarly «p-
on anoíber,
ought not to
be rcfrafled
at all in en*
tringhiío it»

10 . Ayi
am^U the
JAotíon of a
refracicd 5o-
Ay.

Tab. III.

Fig- I.

iii How Rí-
frfílkn U
made.

Tab. III.
Fig.2.

ROHAULT^s 'Partí

10. 7 he ¿xaft Quantity of rhe ReTaction of a Body
paffiiig obliquely out of one Médium into another, may
be decermined, provided we know how much the one Me-
dijin relills ics paíEng more thintheother. Suppofe, fot
ínil.ince, the Line CD feparates the two Médiums, the
upp Toneof which is Air, and the under one Water, and
thai the W ater reüíls the Motion of the Ball A twice as
much as the Air; then let us imagine, that this Ball has
run rhe Length of the Line AB vvith fuch a Velodty, as
takei up a .Vlmute, and is then ready to enter the Water
obliquely : and that theThing may be the eafier apprehend-
ed, we meddle not with whát might happen on the ac-
count of the Bignefs or Weight of the Ball. Letus ima-
gine further, that its Motion in the Air has been all along
unifofm, and that after having loft half its Velodty by
meeting with the Superficies of the Water, it lofes no more,
though it finks never fo deep ; for the Deviation * is
madeonlyin the Superficies, and the Water which refi,|j
all its París equally, can only make the Ball take up more
or lefs Time in moving through a given Line, and not
caufeit to moveoutof it.

1 1. This being fuppoíed ; having deferibed a Circle on
the Center B, and the Diftance AB, let us ílinfider, that
the Ball having taken up a Minute of Time in moving
from the Circumference of the Cjprle to the Center,
where it lofes half its Velocity, ought afterwards to take
up two Minutes in moving from the Center to any Point
in the Circumference : Now in order to determine where
this Point ought to be, we obferve ,• that though the Mo-
tion of this Ball was fuppofed to be a fimple Motion, yet
its Determination in the Line AB, with refped to the Su-
perficies of the Water, is really compofed of two Deter-
minations, one of which caufes it to move from the Left
to the Right, the Length contained between the Lines
AF and BG, which are perpendicular to the Superficies
of the Water, thatis, the Length of the Line AG or F 6 ;
the other Determination makes it delcend downwards the
Length contained between the two Parallels AG, CD,
thatis, the Length of the Line AF. We muft further

traíted into it filf ; which is, becauie
when welook opon any Thing in
the Water, it feems to be nearer us
than it really is ; fo that herein they
falfely aferibed that to Reírañion
(oí which there is none in the per-
pendicular) which was to be aferi-
bed to the diverging of oblique Rays
after Refraílion, . from the Point

neareíl to us. But for the and
manifeft Refrañion of .perpemifeu-
lar Rays, which is made in IJland
ChryfiaU See N'ewt. Opt, pag. azy.

2 Is made only in the Stiperfides)
It is otherwifé in the Reflexión and
Refradtion of Light, See hela'»,
Chap. xxvii, .An. jy. 37.

obferve.

Ig. o/ N ATÜRAL PhÍlOSÓPH Y.’ 95

obferve, that the Superficies of the Water ;refifts the De-
terminatioii downwards, which confequently mufi: be al-
tered; ^ but it maíces no Refiftance at all to the Deter-
minatibn from Left to Right, wherefore this will not be
at all altered, but the Ball which moved in this manner
the Lengch FB during rhe Minute which it took up in
going from the Circumference of the Circle to the Cen-
ter, ought tomove twice this Length in two Minutes,
in going from the Center to the Circumferetice : ’ Let
BL therefore be taken equal to twice -BF, and the Line
ELM drawn perpendicular to CD, and the Ball ought
to be found fomewhere in this- Line, two Minutes after
it has parted firom B ; but it was before faid, that it ought
at the fame time to be in the Circumference of the Cir-
cle alfoj whence weconclude, that the Ball will be at the
fame time in this Line, and alfo in the Circuthference of
the Circle i thatis, in the Point M, where they interfed:

^ch other. So that inftead of continuing its Courfe in
the Line AB produced to N, it will be carried along the
Line BM, which is from the Ferfendicular, * and the
Refradion will be meafured by the Angle MBN. From
what has tsen faid, it is plain, that if thelower Médium
had refifted¥he Ball kfs than the upper one, the Refradion
ought to have been contrary, that is, to the Perpen-
dicular. . ♦ '

12. Without alteringany Thing before fuppofedas to n. ne dif-
the DifFerence of the Refiftance of the two
and the Velocity of the Ball, let us now fuppofe, thzt faiu v‘r} tb-
the Ball, in order to go to the Point B, comes from ano-
ther Point more diftant from the Point P than was fup-
pofed in the former Example, fo that the Line FB which
is the Meafure of the Determination towards' the right
Handhe longer thanhalf the Radiusof the Circle, and con-
fequently the Line BL, which is twice as long, be longer
than the whole Radius; it ought to follow, according to
the foregoing Reafoning, that the Line ELM will fall with-,
out the Circle, and not interfed it at all ; And fo ouc

Tab. III.
Fig. 5.

I. 'Bnt it mabes no Rejtjlancs)
Fut it does refiíl that Determination
alf' 5 as it enters ; for the Ball in en-
tríng, ftrikes or rubs againft the far-
ther part of the Hole which it enters
into : for which rcafon> and becaufe
the Motion of the Ball is afterwards
perpetually retarded as it pafles
through the Water which reGfts iti
ihis Inftanceis not fníEciently accom-
modated to expiara accurately and

mathematically the Nature of Re-
fraftion.

z. And the Refraáiion mil be mea-
fttred) It is a right Obfér-
vation of Cartee here. that Tab, III.
RefracEfion and Fig. 2,

in all Incidencies is to be
meafured by the Propprtion of the
Lines AG and OM, and not by the
Angles ABGi and,HBM, or NBM.
Set Carta’ t Dioftr. Cha¡. a. Art. 7.

Ar-

96

13 » That a
*Body which
falls too ob-
liíjue lipón
noibíTi aught
not to pene-
trate ít at
sil.

i. That the
jPoJition of
hard "Bodies
jpKt inte Li^
^uors is an
EffeS of
Motion,

z, That the
Síiperjicies of
a heavy Li-
tyiior co'fttain-
ed iri a Vef-
J'cly oítght to
be levtl,

Tab. III.
Rg. 4.

ROH AULT^ s System Partí.

Argument feems to conclude, that the Ball ought to be
in two different Places at the lame time, viz,. in this Line,
and in che Circumference of the Gírele j which is im-
poffible.

13. It muíl be confeíTed, that hete is fome Miílalce,
whencefoever ic ariles ; forevery Argument that leads toan
Impoffibility, is defedtive either as to the Form or as to
the Matter of it. But let us not imagine that there is
any Faiilt in che Form of this Argument which feems to
conclude in an Impoílibility ,• let us rather fay, that ic be-
■ing conclufive, it is a cercain Sigo, that the Fault was in
fome ofthe Suppoíitions that were made. Andíoindeed
ic was, for we fuppofed that the Ball, when it had loft
half of its Mocion by meecing che Superficies of the Wa-
ter, would enter inco it, though ic fell never fo oblique,
which is not fo. For we fee by Experience in a Sea-
Fighc, that Cannon-Balls Which are íhot too oblique upon
the Water, are refledted by the Superficies of the Sea, and
kill the Soldiers upon the Decks of the oppofite Ship#
And we oblerve che fame Thing in Stones which Chil-
dren make Ducks and Drakes with in the Water.

C H A P.

Of hard Bodies put into Liquors.

A L L than can be faid of the Place which a Body óught
to poCfeís in any Liquor according as ic is more or
lefs heavy, does properly belong to the Doctrine of Mo-
ción. For chefe Bodies are in Motion when they fink in
the Liquor, and they are in Motion alfo when they rife
from the Botcom, to the Superficies.

2. That we may notpafs by any Thing therefore which
maybeofufe afcerwards, let ABCD be aTub filled with
Water, and fuppofe firfl., that this Water is upon che Le-
vel, that is, no one Pare of the Surface AD higher than
another,- then imagining it to be divided inco a great ma-
ny Columns, perpendicular to the Bottom of che Tub,
let US examine one of chafe Columns, as EFGH. And
firíl ic is obfervable, that though this whole Column en-
deavours to fink down, yet it cannot, becaufe the ímal-
1er Columns, into which this may be fubdivided, muft
bend at theBortora of the Vellel before they can return

up-

Chap. i6. of Natural PHiLosoPHy.

upwards ¿ but thac they cannoc do, becauíe they meet and
fupporteach other, and arealfo fupporced bythelittle Co-
lumna on all Sides of them^ which cend downwards like-
wife, and with equal Forcé. So that the Water in the
Tub ought co continué ' upon the Level, and to remain in
perfed: Reft zná Mquilihrio, if there be nothing elfe buc
its own Weighc to move or íhake it. Whence it is ma-
nifeíl, that if we fuppofe the Water in the Tub to be
higher in one Place chan in another, that it cannot conti-
nué foj becaufe thofe little Columna of Water which are
longer than the other, will have more Power to defcend
than they, and will never leave crouding them up^ till the
Surfaceof the Liquoria come toa Level, when they will
all be in a^uiUbrio with each other. Therefore when a .
heavy Liquor ia contained in any VeíTel, ,we are to think
that ita Weight diípofea the Surface of it to be upon the
Leyel, and that it will continué fo, unleía altered by fome
Jforeign Caufe.

® 3 . Let ua confider further, that if there be put into the s- That »
Water in thia Tub any hard Body, fuch aa 1, of equal
Gravity with the Water; as its Weight would have 'Ci€\-Uq,-.or of e-
ther moremor lela EfFed; than the Water whole Place it ^•“‘‘Gravity,
poíTeíTes ; 'Siete is no Reafon why any Alteration fhouídbe
made in the Column EFGH, fo that the Body I muít »/»>.'
cpntinue where ir^as placed.

4 . But if we ilmgine thia Body to be heavier, by an A-wuhwhat

Ounce, fuppofe, than a Quantity of Water of equal Bulle; 77hkh'’iTheI
it is manifeft then, that all the Columna of VVater will vier thanWa-
not be in ¡sfálibrio, but the Body will go to the Bottom, to

not with its ordinary Weight, but only with the

rence betwixt that and the Weight of a Quantity of
Water of equal Bulk, thatis, with the Forcé of an Ounce
Weight.

5 . But íínce Water was here taken only for an Exam- ;■ That we
pie, and the Reafoning holds the fame, when applied to

any other heavy Liquor ,• we may affirm in general, that Weígltofa-
in fupporting a heavy Body, we ought only to feel the Tody by

Excefs of its- Weight above that of an equal Bulk of the Atufes.

Liquor in which it is. Henee it is, that we are not fur-
prifed to find by Experience, that a pretty lufty young
Man who weighs a Hundred and thirty eight Pound in
the Air, does not weigh above eight Ounces in the Wa-
ter. But v/e have before íhown, by many Experimenta,

I. Upon ths Level) That isjas to
Senfe. But in realiiy it is pan of

the Sphsrical
Eatth.

H

Superficies of the

that

98 ROH AUL'í’s System Partí,

that the Air itfelf is heavy, wherefore we do not by our
Senfes feel the true Weight of a Body iii the Air^ but on-
ly the DifFerence of the Weight of the Body and of the
Air ; and confeqaently, unleís we are under any particu-
lar Indifpofition, we ought never to feel ourfelves lighter,
but only when the Air is heavier.

6. That a Ic is cvident, that if the Body L juñ now mention-
•Sodywhhh edj had been fuppofed lighter than the Bulk of Water^

whofe Place it poíTeíTes ; the Column EFGH would not
ought ro hfc be heavy euough to be in tsquilihrio with the reíl of the
Water in the Tub ; wherefore this Column will be forced
^orceT‘ to gtve way, till the Body I be got up to the Surface
Tab. III. AD, beneath which, fo much of it wsll remain, as pof-
f'S- 4- fefles the Place of a Quantity of Water equal in Weight
to the Body.

7. Hom to 7. From what has been íáidj we draw two very

^‘L7d‘ild 'tnportant and ufeful Inferences. Firíl, Ihat if a Body
«sigáis morí ‘Tny Líq^uoT, jiTtks to tíoe Bottom, it is certain t¡o^)

tr iefs than Body is heavier, than an etyual Bulk of the Lñquor, hut
2 -ill”ofati fosims 071 the Top, it is an infallible Sign, that it is

Liquor. lighter.

8. Tht nay 8. Sccondlyj If a hard Body he put hito /.¿uo Liquori,
to find rnhuh and rifes in the orne, hut fnks in the other, the fornter
‘oftm^Li-^'^ necejfarily he . heavier than the latter *,

í-. 9. This

* It is worth while to explaín, a
ItEcle* more fully, and in betcer Or-
den, ’^he Hydroíiatick Propofitions,
•which are urged too briefly and con-
fufedly in thisChapjer.

I Thereioveuíliy’f^ate.r^ravitates

íti every Vlace-, cven in V^ater it ■ fclf
(and che faméis to be undeifíood oí
any ocher Liquor) and by reajm oj the
ce¡íuiL frejjísre of its Parts orí all
SideSi its Superfiíiies ought tobepUin
andleveU This is demonflrated -in
the fecond Article of this Chapter,
and by the famous Mr. “Soyle in his
Hydrofiaikkso Paradox i.

2, A hard Hody^ fuch as I, eqnal

in VPcight to a ^at t'fy
Tab. m, of FPater of the fame
Fig. 4* iulk, pTit into FFater,
ought neither to fink ñor
rifci but to refi in any Place. For
the Column EFGH gravitares nei-
ther more ñor lefs than the Columns
which furround it, and cherefore it
ought to keep in aquilibrio. See
Art, 3. of this Chap,

3. A Body ^ fuch as heavier than
VFater, ought ío fink ;« the Wa-

ter, Becaufe tben the Column EFGH
is heavier than rhe Columns which
furround it. See Art* 4. of this
Chap, .

4. AffBodyt fuch as I, heavier than
Waterf\ anght to have jufi fo much
Weight in Water, as it exceeds in
Weight an equal Bnlk of Water,
For fince che Body I poííeíTes the
Place of an equal Bulk of Water in
che' Column EFGH ; it is manifeft»
chatbyhow much that Body exceeds
Chat equal Bulk of Water in V/eigbt,
by juñ fo much is that Colurnn hea-
vier than ic was before. See Art. 4.
of this Chapter, and Archimedes e/
’Bodies put hito Fliiids. Prap. 7.

Henee, fmee the Proportíon of
Weight betwixc Gold and Water is
known, Gold may be proved and
valued, by Weighingic in Water. See
"Boyle’s Hydrafiatick Medicine.

y. Any Body fuch as I, put inta
Water, is mt only preffcd doxonroarás
by the inenmbent Water, buc is alfo
prefl'ed upwar ds by che Water ihat
is under ic, This is evidenc from
firfi Propofition. See alfo Soyle’s Hy-
drojiaticksf Paradox 3, 6 , Tht

Chap. 1^. ^ Natural Philosophy.

99

9. This being fo, if we examine the Opinión of fome
Philofophersj viz. cliac there are certain Places natural to
all Bodies where they of chemfelves continué at reñ^ and'° ^
have no Tendency to go out of them^ and that this is the

Rea-

The heavleji 'B-)dy of all, fnch
as I> a Cube of Goldi if it be put
fo deep into the IVater, that tht
Depth of the bVater from . i-1 to thi
lovoer part of that Cnbc bt tveenty
times as mitch as the Thicbnefs of ¡
iSy that Cube x$sll be foprejfed up
reards by the Water that is nnder
;í, that^ if the incítmb:nt Water
KÍH voere removed, it xootddnot Jsuk.
ForfincetheCubel isjuftüfche fame
Weight as che Water which reaches
from EH cü cheBoicom ofche Cube .
all which Water we now ruppofi;
to be removed; ic is evídent, tbar
ihe Column FIG in this Cafe, is
a^uilibrio wíth che Columns which
furroundic, and cherefore the Cube 1
cannot íink, See Hydrofatick Ta
rad. II,

7. ^ 'Body, fíteh as I> lighter than

Witter, letf^t be prcjfed never fo
mnch by the'^nmbent Water, ought
to rife notvoithjianding* For in rhís
Cafe, the Column EFGH is lighter
than the Columns of ^ater which
furround it. See of this

Chap,

8. When a light *Body is rifen to the
Topofthe Water, fo rntteh of it ought
to remain under the Water^ as is
e^nal to a Unlk of Water tveighing
as much as the vohole "Bcdy, Thisis
the Fifch Propoficion of Archimedes

conccrntng "Bodies put into
Tab. III. Flnids, and is eafiiy de-
tig. 4. monftrated from wliat has
been airead/ íáid. Foric
is manifeíl, that when che lower
Pare of the Body rwimming in che
Water, is funk in this Proporción,
the whole Cohmn EFGH is in a-
with the Columns that fur-
round ic ; and if che fame Body be
funk deepefs this Column will be
lighter than chereft of che Columns;
if noc fo deep, ic will be heavier.

p. In every Body that is lighter
than Water,, the. Proporí/o« of its
Weight to the Weight of Water,
is as that part of it nnder the Wa~
itr to the vohole Body. This Propoíí-
cion follows fróm che preceedingone,
and is more ar largedemoníiraced by
Archimedes, Book !!• Prop, of Bodies
put into Fluids,

TO. All Wa'er prejfes itpPn the
Bodies under it, in proporrion to its
perpendicular bieight^ and not mpro*
oortion to its Breadth. This noble
PffpoGcion is at large demonílrated
in my Notes upan Chap. i o. Art. ir*

í I . This Prejfure • afls upan Bodies
immerfed in the Water, not only on
the Top, bnt on the Bottom and the
StdeSi every voay equally^ This Pro-
poficion follows from che foregoing
one, and is demonílraced from che
N.ture of Water, whereby every
Preílu'e is propagated equally and
entire every way. See aifo Boyle's
Hydrnfi, Paradox 7.

12. Henee, awooden l'rencher put
under Water, immediately rifes up ;
íhouiih there be a mitch greater pistan-
lity of Water lying above it, than
is under it i neither is there any fuch
Thing in Natnre as Levity, to lift
it up. This Propofiiion you have
demonfiraced in my Notes on Chap.
X. Art, II. Coroll. 3.

13. However, If the xoooden
Trencher be exañlyfittedto the Width
of the yejfel, fo that no Water can
gtt in between it and the Sides of the
Féjfel, tohich by communtcating its
Weight to the VFater beneath, miglot
forcé the Trencher up ; or if the
Trencher gocs fo clofe to the Bettorn of
the Feffcl, that no VFater can gct in
bettoixt it and the Bottom, then the
Trencher xoill not rife at all. Which
is a manifeíl Proof, that there is no
fuch Thing as Levity in Nacure.
See the fame Place,

Ic is very hard to prove chis Pro-
pofuion by Experiments, becaufe
Water isfo apc to wec and run all
abone. But 1 have tried it with
Quickfilver, which will noc wet moft
Bodies; for afeer I had genrly puc
a Piece ot Money on the Bottom of
a Veflél full of Quickfilver , che Mo-
ney did noc rile up ; but if I íbakcd
che Veflel, or lifted up the Money
ever folitcie with a Ñtedle, chatíbme
of che Quickfilver mighc gec be-
cvvixc che Money and the Bottom
of the Veflel, the Money was im-
mediacely raifed up.

14./Í

H 2

loo

ROHAULT’s System Partí.

Reafonwhy Water has no Weightin Water,- wefhallnot
fcruple to affirm, that this is as grofs an Errour, as it
would be in a Man, who, feeing a large Canon in bne
Scale, and feven or eight thoufand Pound Weight in the
other, íhould afErra, that the Canon did not weigh any
Thing in this Place, becaufe he can eaGIy lift it up or
down ; For this Opinión of thefe Philofophers is founded
upon thisExperimentj that indrawing Water outof aWeIl,
we do not begin to feel the Weight of that with which
the Bucket is filled, till it comes into the Air ; whereas
they oughc to think, that as the Canon is always hea-
vy, and we could not eafily lift it, but for the Weight
which keeps itin aquilihrio ■, fo alfo the Water weighs al-
ways the fame ; and the Reaíbn why we don’t perceive
its Weight when the Bucket is under Water in the
Well, is, becaufe we are afliñed by the reft of the Wa-
ter in the Well, which is in ai^mübrio with that in the
Bucket. (j

14. It is piijjible for Water to de-
prefs and Jtnk a "Bady light-
Tab. I. er than it felf. This may be
Fig.4. done by gently putting che
Syphon ABCDa filled with
Oyl as high as ABC, into the W ater dll
che /horter Arm AB be under Water i
for then che Water.preifingupon the
Superficies AB, will lift up che Oyl
fo much the higher towards D, astíe
Syphon is lee down deeper intoit.
And from henee alfo it is as clear as
the Sun at Noon-Day, that there is
no fuch Thing as Levity in Nacure.
But lefi che Experimenc Ihould fail

by the Oyl’s mixing with che Wa-
ter, it is more proper to ufe a Syphon
with fmaller Arms. See BoyhU Zth
Hydrofiatick Paradox, fp.

ly. So likewiíe, it\nay he, that
Oyl having Water on each Side of it
may not rife up) viz, thusj if, when
the Syphon^* filled with Water up
to ABC, Oyl be pourecf upon che
Water in each Arm, and Water be
again puured upon that Oil to baU
lance che Prefíiire of che lower Wa-
cer upwards. See Boyle's Uyilrofia^
ücksi Paradox y.

CHAP.

101

Chap^ i7 ' Natural Philosopha^.

ig® Ig® ©Í5 Ig® (®a (giS)

C H A P. XVIL

Of Accretion, Diminution, and AUeration.

I. Vj^haiis
tneantby Ac~
etetion and

Á S Ar 'tjfotle in treating of local Motion, confiders alfo
the other Changes thac happen to natural Bodiesj
fuch as Accretion, Diminution and Alteratmi, which he Dimnmtim.
calis Motion likewife j fo we after his Examplcj íhall not
wholly negled thefe, but íhow that ir was not without
Reafon, that he brought them under this Head^ fince
they are indeed the Eífeóts of local Motion. _AI 1 the
World agree^ Chat by Mccretion and Diminution is meant
the fenfible Increafe or Decreafe of the proper Subftance'
of a Body ; Thus we are fure, that the Trunk of a
Tree is increafed when we fee it bigger than it was be-
fóte.

3. Since we obferve, that Trees, and in general all Hí» 2t-
Bodies ftand in need of NourilEment, to make them fdeafil a«d
increafejl|and that it is impoffible to conceive how a Sminiíhed.
Body íhó'uld increafe and becomc bigger without fome
Parts being added to its former Bigneís j this is a con-
vincing Proofj •'^t every Body which increafesj receives
fome Augmentation of Matter. And as this is true of a
Body which increafesj lb_, may we alfo affirm^ that eve-
ry Body which decreafes, lofes íbme of the Matter which
it had befóte.

3. However this does not hinder us from making a ■^.Thatin-
difference betwixt Increafe and Rarefañion ; and betwixc
Decreafe and Conde?ifation : For the Matter which is ad- Rarefaíiim.
ded to a Body increaíing, and That which is taken from
a Body decreafingj is looked upon as belonging to it,
and as part of its proper Subíiance j but, as was befóte
obferved, the Matter which entersinto the Pores of a Bo-
dy to rarify it, or That which gets out of its Pores, thac '
it may be condenfed, is looked upon as Matter that does
not belong to it.

4. The Idea we have of the Accretion of a Tree, be- ^-Thiit thire
ing different from the Idea we have of its being. tranf-
plantedj it muft be owned, that Ariftotle had Reafon to feme be-
make a diíference betwixt Accretion and local Motmi.

However, as a Tree cannot be tranfplanted, but by the d^/mdth 7
local Motion of its whole Body, fo we cannot conceive local Uotion
how it ílrould increafe but by the local Motion and
H 3 Union

102

5'. fío» !Bo-
áUs are al-
uredf

i.Thatforms
'pHgbt to he
tveated of by
títetnjelves t

ROHJULT's System Partí..

Union of the fmall Partióles which contribute to the in-
creaíing it.

5 . When a Body neither increafes ñor decreafeSj but
is fomewhat changed, it chis Change be not fo great that
we do not at ali know it ; we cail it, as was faid before,
Alteration \ henee it is eaCy to fee; that there can be no
Alteration. without ¡ocal Motion ; For how can there be
any Change in a Body, if none of the Parts which com-
pofe it, and npon the particular Order of which ics Nature
depends, have changed their Situation ? This being fo, it ^
is very evident, that there mufl; be aii Alteration in a Bo-
dy, wh-en the fenfible or infenfible Partióles of which it
h compofed, are puc out of their Order, or any great
Change made in their' Figure : Or it tnay alió fuíFer an
Alteration^ by the Acquifition of fome new Partióles, or
by the Lofs of fome of its oíd ones^ all which cannot be
without local Motion; Thus, when there is an Alteration
in a bruifed Apple, we can eafily imagine that many ofÍ!
its Particles have been forced to change their Situation,
and perhaps fome of them have alfo changed their Fi-
gure. If after this, any one üill doubts whether there
may not be fome kind of Alteration in whic^'; there is,
fomething elfe belides what proceeds from local Motion, I
think he cannot be fatisfied better, than by what we are
now going to fay of Formt.

>

C H A P. XVIIL
Of F o R M

J fORMS are a Subjedt that we cannot hope to treat
of, as we have done of Matter. For lince Matter
is a commpn Suhfiratum, which, when once we underftand
what iris in Wood, we cannot at the fametime but un-
derftand what it is jn Fire, and in every Thing elfe,- one
{ingle Refledtion is of it felf fufncient to gain che Know-
ledge of it. But becaufe the Form of any 1 hing, is that
which makes it to be that particular Thing, and diftin-
guifhes it trom every Thing elfe ; it does not follow, that
}f we know the Form of Wood, we therefore know the
Form of Fire, or any Thing elfe. Wherefore if we
would fucceed herein, and fay fomething more than ordi-
míYf we muft defpencj fo Partieiflafs, nocwithftariding the

C«-

103

Chap. iB. Natural Philosophy.

Cuftom of PhilofopherSj who feldom do fo, but for the
moft part contení themfelves with propofing abundance
of looíb Queftionsj which we may look upon as fuperflu-
ousj and from which we can gain no Advantage.

2. Hovv'even I do not affirm^ that ic is an ufelefs En- y;
quiryj if it fhould be asked here^ as ufually ic isj whecher Fcrms, and
there be any fuch Things as Suhfiantial Fornis, that isj that the la-
Form which arft real Suhft anees j and confequently have a
diftind Exiílence frotn that of Matter. But thus much d«es mt
ac leaft, I may venture co affirm ; that the Solution of chis f”®*
Difificulty, dependa upon the particular Knowledge. of
the Things. The Inítance of the rational Saúl proves no-
thing here ; for though we know that chis is a Subftance
really diftind from the Bodyj to which it is united, and
that ic does not at all depend upon it for ics Exiílence,
yet we can conclude nothing from henee as to the Forms
of other Beings which are pu^dy ■ material.

_ 3. But if we coníider this Matcer more clofely,' though 3-.

T acknowledge, as all the World do, that the Soul is that \Tn”t\he'‘^
which particularly makes a Man to be a Man ^ and con* Farm of the
fequently that it is truly the Form of a humane Body as
humane i xec I can’t agree, that it is, properiyípeaking, the
Form of S. that which is íénfible, and is called the Body
and coníidered limply as a Body ; any more than it is the
Form of any of^s Parts, coníidered as diíFerent from
each other : For m this Senfe, every one of them has its
particular Form fo clofely conneded with the Matter of
ic, that ic continúes as long as the Part fubfiíls, even af-
ter the Soul is feparated from che Body. And indeed af-
ter fuch Separación,' every pare appears the’fame, as it did
immediately befóte. For, that which was Fleíh, for In-
ílance, is Flefli ílill, and that which was Bone, is Bone
ílill, and fo of the reíl.

4. The Caufe of many People’s Miñake, who con- 4. aín Error
found the Properties of che Body with thofe of the Soul, -P**"
ischis; that a dead Body, when the Soul is feparated from^'’’^”'^
iCj is uncapable of many Fundions which we obferved in
it befgre, fuch as moving ic íelf, Refpiration, Nouriífr-
ment, fo that they perfwade themfelves that all thefe
Things depend upon the Sou!, and would not have ceafed
in the Body, if the Soul had not deparced from it ;

Whereas we ought rather to think, that the continuing
of che Soul in the Body, depends in íbme meafure upon
the Diípoíition of the Body to perform theíe Fundions,
and that the Separación is a Confequence of thefe Fundi-
ons not being able to be performed. For every Day’s
H 4 Ex-

104

y. That thcre
are ejfential
^ormst

ۥ That h ts
not ccrtain
that Ariílotle
did allovo of
fnbJiaTitial
■ Fornis.

7. That
j^nijidal
Forms are al-
fa natural.

RO H AUL'T’s Pare I.

Experience fliowsus, thatDeath never comes, ñor is the
Soul ever feparated from the Body, till ir is fome way
hurt, or by fome Means Ipoiled and corrupted. And we
have no Example of the Soul’s being feparated from 3
found and perfed: Body, and that this Body did not be-
gin to be corrupted, till after, and becaufe the Soul was
léparated from it.

5. It would therefore be unreafonable, upon the íingle
Inñance of the Rational Soul, 'whlch is very dlfférent frotri
the common Forms of Bodies, and without firft knowing
the-particular Form of all Kinds of Bodies, to aíErm here
raíhly, that there are fubfiantial ‘Form in Things merely
corporeal ; however we may venture fafely and confi-
dently to aíTert, that there are fome Forms whlch are ef-
fential, that is, fuch as belongneceíTaríly to theirSubjeóts :
Thus to he liquid is eílential to Water, becaufe there is no
Water whlch is not liquid.; we may alfo affirm, that
there are other Forms which are only accidental, becaufe^,
they fo belong to the fubjeci:, that it can exifl: without
them, and not ceafe to be what it was. Thus coidnefs
is an accidental Form of Water, becaufe Water would
ftill be Water, if it was made hot.

6 . It might very eafily be, that Ariflotle ’might ac-
knowledgé ejjéntial Forms and not fuhflantial Forms ,• for
it is certaín, that the Greek Word wlí"ih he ufes, may
as well or better fignify the one than the other.

7. Forms are commonly diftlnguilhed inío Natural má

Art^cial: They aW úioíe Natural, which belong to tha
Subject without the Affiírance of Men ; Thus a Portion
of Matter réteives the Form of Marble in the
Bowels of the Earth. Artificial Forms are thofe that
proceed from Art; thus the Form of a Clock is
called Artificial, becaufe it is owing to the Labour of
the Clock-maker. I agree, that if the Ñame had been
given with regard only to the Caufes by which they were
produced, it would have been reafonable to cali tíre one
Natural, and the other Artificial j but fince it is inferred
from thence, that the Fomr are different from the

Artificial Forms, and that they act from internal Prin-
cipies, which are very different from rhofe of Artificial
Forms- there lies the Mifrake. For Artfidal Forms are
as natural as the Natural Forms themfelves, becaufe they
proceed from Caufes purely natural ,• and Art, as was
iaid before, does nothing elfe but apply adive Things tq
oaffive Ones.

Chap. I9‘ p/ Natural Philosophy. 105

8. Ic is much more reafonable to divide Forms into
Simple Compomid. Simple Forms are thofe of fimple
Beings, thac is^ of Beings thac are capable of but a und com-
few Properties^j - and compound Forms are thofe of com-
pound Beings, that is, Beings that are capable of a great
many Properties. For Inftance, the Form of a hard Bo-
dy, whatever thac Form may be, is a/w^/e Form compared
with the Form of Wood, which, wich refpeít to the for-
mer, ' may be faid to be compounded ^ becaufe a hard Bo-
dy, as hard, is not capable of fo many Properties as
Wbod.

p. This Obfervation is more remarkable than one^.Thatfm-
would imasine. For ic is evident, thac (Imple Things may
beknown, whenwedont at all know thole thac are com^ «nderftaoi
pouhded of them : Whereas we cannot know thofe that firp.
sxt compounded, but we muft have a diñindt Knowledge
of thofe Things which go towards their Compofición.

Wherefore in order to underftand particularly the Forms
of Bodies, it is neceílary thac we firft begin with thofe that
are Jlmple, and afcerwards come to thofe thac are com-
pounded.

--m

CHAP. XIX.

Of Elenients according to the Opinión of the
Antients,

TF we once have a clear Notion of what Philofophers
•*-mean by the Word Element, we cannot doubt, but E-

that the Forms of Elements are the moft fimple of all. tcments.

Ir is to be obferv’d therefore, that the principal Defign
of Philofophers is to explain how every Thing is gene-
rated, in fuch a manner as to let us know the difFerent
States through which fuch Things pafs from their firft
Principies till they areentirely compleat, and in that perfedt
State in which we fee t^em. And in order to this, fince
they find by Experience, that every Thing is not made
indifferently out.of another, and that Stones, for Inftance,
and Marble are not proper to be converted into Fleíhj
neither will they ferve to nourifh it and make ic gfow j lo
theyjudge by proportion, that all forts of Bodies are not
compounded of Principies alone, connedfed together in
che moíl limpie manner pofíible. j but only fome very limpie

Things

io6 RO H yi ULT’s System Partí.

ThingSj of the Mixture of which all other Things
are afcerwards compofed. Thefe very fimple Things^
whatever they be, which thus arife frora the nrfl: De-
terminatíon and Connexion of Principies, are what Phi-
lofophers cali Elemenls : So ■ that Elements difFer from
Principies in this, that a Principie, fuch- as Matter, for Éx-
amplcj is, as it were, an ■ incompleat and undetermined
Tloitíg-, whereas an Elemesit, is a compleat and determined
Thmg.

2. This being explained, theremuíí:, withoutdoubt, be

mfre E/e- Eleme?it, otherwife there would be but one

mentí th^n unifortn Simplicity inNature, and no compounded Things.
ThiOpiJmof Philofophers have not agreed what is meant by Ele-
tue Antients ment'^ thc Rcafon of which is, becaufe they have not fo
mas concern- much ioquircd into tloB Nat/ire of Thmgs themfelves as into
m¿ emen s. Senfations luloich they are apt to raije in ns. Thus foine

Philofophers who coníidered che Seníe of Seeing only, have
aííerted that Light and Dark, Tra'nfparcnt and Opacous were
the Elements of Things. And ochers, who referred ev®
ry Thing to Eeeling, have pretended that Hard and Li-
quid^ or Hot and Coid were che Elements.

3. may be placed amongd: che NutnberOf thefe
FonrEfe- ‘ though he went in a Way fomewhat diílerent frorn
mentí. theirs. He coníidered firft, the principal Qualities that

come under the Senfe oíFeeling, fuctua^ Heat, Coid, .Dry-
nefs or Hardnefs, and Moijlnefs or ió'fuidnefs : And afcer
he had obferved that two of thefe C^ialities might meet
in the fame Subjeít, and that the Four might be coupled
four different Ways, he compofed four Eletrftnts ; of
which che Firíl is Coid and Dry, the Second is Coid
and Moijl, the Tbird, Hat and Moijl, and the Foürth, Hot
and Dry.

i.n'hat 4. Then, in order to give Ñames to them, heexamincd
ílra. Things in Nature were, in which one Ele-

ment feemed to prevail, or in which its - Qualities were
moíl fenfihle. Thus, imagining the Earth to be both
the coldeff and drieíl Thing in the World, hecalled his
Eirfl Elenmtt, Earth. So likewife, becaufe he thought
that Water was the coldefl: and moifteíl: Thing, be call-
ed his Secoézd Element Water. Further, imagining alfó,
that there is nothing more moifl: and hot than Air, he
called his Third Element, Air ; And laílly, not doubting,
but that Fireis the hotceftand dryeít Thing inthe World,
he called his Fo'urth Element, Fire,

5. Ari-

Chap. 19 *. of Arv^Az Philqsophy. 107

5 . Ariftotles making ufe of Ñames which were before s- Thatthtft
ufad to lignify otherThings, hath given occafiontomany,
whodid noc rightly apprehend his Meaning^ weakly to if/o»,,.
believe, that Thls Earth which we inhahit, This' Water

which we drink, This Air which we breathe, and This Tire
■which we kindle, are the Four Elements. But chis will appear a
very grofs Miftake, to any one who confiders, that the
Ñame Ekme7st is given only to the mofi- fmple Eodjy
whercas che four now mentioned |ire the moft compound-
ed of any we know.

6. But if we fuppofe the Ekments of Ariflotle to be as

fimple as he makes them, and if we compare them wich jiM^ldXy
thoíe which ocher Philofophers have atcempced to intro- AriiiotieW
duce ; we do not find any Advarrtage they have, why we
Ihould prefer them above others ; becaufe in this Matter cehcd.
we have no more reafon' to coníider the Qualicies of Eeel-
ing, than thole of Seemg^ or any other Seníé. But nei-
'^r che one ñor the ocher oughc to be allowed, and that
for the fe two Reafons, which feem to me very ftrong.

TheFirft is, That in order to eftabliflt Ekments through-
ly, ic oughc to be upon the Eeterminations which may
happen to ^latter abfolutely and in it felf, and not upon
the Eelations which the diflferent Forms of which ic is
capable may have to ourFaculcies to raiíé Senfation. The
Second is, that alllütefe pretended Elements being decer-
mined by fenfible Qualicies, of which we have no clear
Notion ; íc is impolfible, but chatthere muíl remain fome
Obfcuricy, into which no Phüofopher can fo far pene-
trate as to be able to fee what will arife from their Mix-
ture j in che fame manner as a Phyíician cannot tell what
is the Vercue of a Medicine compofed of many {im-
ple ones, of which he has only a confufed Know-
ledge.

&

CHAP,

io8 ROHAUL'T’s System Partí.

C H A P. XX.

Of the Elements of the Chymijis.

1. The Mt- T whether cheíé or fuch like Reafons, in-

thori of the ■*- duccd the Chymijis to reje£t thofe Elements which the

¿a Antients would have introduced ,• thus much is certain,
Eiémtnul f^ac they had propofed others very diííerent. And in
order to eftablifli them ; as they profefs an Art which
' coníifts principally in ufing Fire after diíFerent manners,
to feparace as much as is poílible, the diíFerent Parts of
W’hich diíFerent Bodies are compofedj they have pretend-
ed, thatthis Refoludon is the only Way tofind our what
are the true Elements whieh Nature makes ufe of in the
Compoíition of Bodies j as the taking a Machine ^
Pieces, is the only way to find out what it is compo-
fed of.

2. What the 2. Thus, in working upon certain Bodies, upon Wine,

Mercttry of fuppofe, they put a large Quantity of it into ^ Alembick,
the chymff.s nieans oí Fire, make fome of its Parts exhale,

, which being then condenfed by the Coid, fall down into
anotber Veííél in the Form of a ftro^rr fubtil, and pene-
trating Liquor, to which t-hey are pleafed togive the Ñame
of Mercury, Spirit, or. Aqua-mitcs.
tj.xmiatti 3 - After this, continuing the Alembick upon the Fire,
they make it diílill a Liquor which has no Talle, and
ald Sui^imr. 'Phkgm^ and fo they go on till there re-
mains nothing in the Alembick, but a glutinous Subjiance
like Honey. Then they put this glutinous Subílance in-
to a Retort, and wirh Fire they make it again diílill a
Fhkgm like the former, and then an acid Liquor which
they cali Mercury alfo ; and after that, another Liquor not
quite fo fluid, fomewhat like Oil, and which is inflammable
like it, to which they give the Ñame, Sulphur.

4.mat!tis 4 - Laílly, They take that -which remains in the Retort,
that they cali and which prefently grows dry, -and burn it, and put the
20 Earthen Pot or Pan, wiih a certain Quan-
Salí.' ■ tity of Water, which in a fliort time becomes Salt, then
ílraining it oíF clear into another VeíFel, there remains
in the Pot a kind of duíly infipid Earth, which they cali
Caput mortuum or Terra dam'naia ^ then with a gentle Fire,
they make the clear Water which is in the othcr Veífel
co evapórate intirely, and after that, there recn-ains at the

Bot*

Chap. 20. of Natural Philosophy.

Bottotn of the Veffelya hard brittle Body wfeich is very like
Sale, and therefore they cali it Salt.

y. Henee they conclnde, that thefe five Subílances, viz.
Mercury, Thkgvz, Sulphur, Salt and Caput mortuum, are
the Elemenes of Wine ; And becaufe whatever they can
extraíb ouc of any other Snbjed; refetnbles one or other
of thefe, therefore they conclude in general, that thefe
Things, are the only and the true Elemettts of all the
mixed Bodies which are in the World, and that all the
Variety that we fee is owing to the differenc Mixture of
thefe.

6 . I fhould think it a great Piece of Injuftice not to

give the Chyinifts that Commendation which is due to
iheir Induñryand laborious Application. Without doubc
the whole World, and the Philofophers particularly, are
very much obliged to them for the Pains they have taken,
and which they continué to take, to make a great Num-
hjr.of Experiments, whereby they come to the Know-
ledge of diverfe Properties ofraanydifFerent Things. This
gives them opportunity to find out and difeover the Na-
ture of Things, and at the fame time, ferves for a Rule
R) try the of their Principies by, and to juftify their

Reafoning md the Confequences which they draw from
thence. However I think their manner of treating of Phi-
loíophyis not fatisfr^ry, noDtheir Eknwnts fuchas ought
to be allowed.

7. Though the exceílive Commendations which they

give themfelves, and with which their Books are filled, as
if they were the only Philofophers, and the Secrets of
Nature depofited in their Hands alone j and though the
large Promifes they make, which for the moft are falfe
and vain, have rendredthem almoft univerfally contempti-
ble to the World j and the obfeure Terms, and almoft
perpetual Equivocations which they ufe, have made them
ridiculous alfo to a great many ; Yet Ido not depart frora
their Opinions upon this Account. For as to thefe ex-
ceíTive Commendations, and vain Promifes, they are only
perfonal Faults which any one may eaíily lay afide, and
which fome Chymifts of my Acquaintance are entirely
free fromj whofarfrom beingvain and proud likeothers,
a?e on the contrary, fo modeft, that if they had nothing
elfe to recommend them, they ought upon this Account
to be placed in the Rank of Gentlemen. And as to the
Obfeurity of their Terms, fome of whicli are authorized
by Cuñom, that is eafily difperfed, if we give but our
felves the Trouble to explain them. '

8- That

109

jf. Thaí

cnryy

SulphuriSalt»
and Capuc
mortuum,
are the Ele-
ments of the
Chymijii.

6. Üo"» Chy-^
Tnijiry may be
«feftil to £¿¿»
lofophers»

7 . TheEf-
rour qf the
Chymfí,

no

RO H AU L T’s System Part í.

S.Thatthty g, That whích makes me not to approve of the Me-
^theraUthl of thc Chymifts:, is, firílj becaufe ic is defedivcj
£artí of a for it is certain, that let them take never íb much Pains,

fenüble Parts. ef which
wMch thcy a Body is compofed ; For as tó thofe which refemble that
doget toge~ fubtü MattcF;, the Exiñence of which, we demonílrated
*taed!^ above, and which gp to the Compoíicion of a great ma-
nyThings, thefe efcape all their Pains. But furtber, that
which they give the Ñame of Principie to, cannot but
be very much akered, and very different from what it
was in the Mixture : For it is impoffible, but that the
different Part,s which they Extradt, when they are put in
Agitation by the Fire, and daílied one againfb another,
muft be changed both in their Figure and in their Na-
ture. And this is confirmed by Experience, for if all the
Parts into which the Mixture is refolved, be mixed toge-
ther again, the Refult will not be at all like the formet
Mixture. _

s-That^ai- 9. To th¡s may be added, that the Chymifts decefve
<!/', themfelves, in faying, that there are butfive Elements : Fot
rAe« mihnl allowing of their Method, and the Manner upon which it
iemore'than is foundcd, wemuftfay, that there is a great Npmber, yea
fiveSiemmis. pj gre^t, that it is impoííible to know theiii all. Thus
there areagreatmány Sortsof iííémrr/, Sulphur^ Sah, &c.
But to mention Sait only ; we finc^lmoíl as many dif-
ferent Saks, as there are different Mixtures. For Exam-
ple, That which is extraóted out of an Afh-tree, is
Cauftick, that is, will corrode and burn the Flefh, if ap-
plied to it ; but that which is extradled from an Oak will
notdofo.

lo.Thaithej lo. But that which fhocks me moft in the Reafoning
cmf«f7¿No Cby?ml¡:s, is the Confuíion that they are unwilling

tionof their to get out of, and the Averfion they have to clear and
ormEkvienu. diftinót KnoWledge, which it is fo natural to delire. For
Inílance, if we ask them what they mean by Sulphur,
they will anfwer indeed, that it is a fat mflammahle Suh-
but if we go on to ask what this fat inflammable
Subftance is, which they cali Sulphur, and in what this
Property of being Inflammable confifts, they will not only
not give US any further Anfwer, which indeed is no great
Matter, becaufe they have none to give; but they will be
offended at our Curioflty, and that we íhould have any
Delire to be ílitisfied herein ; So that their Science extends
no further than to give Ñames to- things whofe Natures
they underñand not, and confequently from the Mixture
of which, it is impoffible to foreíee what will arife, whích

is

Chap. 20. o/Naturai.Philosophy. iii

is one of che principal Condicions which we require in
Elements.

11. Perhaps it will be faid here in favour of che it.Thfpre.
Elements of che Chymi(ls, and in favour of thofe of che
Ariflotelians, thac though we do noc know diftindly whac of the chy-
they are in chemfelvesj yet we know at leaft whac they

are capable of;, thac is, che Senfations they raife in us, or
che Convenience or Inconvenience we receive from
them, which they chink fufficienc to determine whac the
Effedof their Mixture will be. For, fay they, we may
lay down two general Rules hereupon ; Firft, That if
two things feparately, are capable of producing the fame
EffeB, they •uiÜl alfa be capable of producing it ‘udoeft they
are mixed together. Secondly, That f Two things Jopa-
rately, are capable of producing tvjo contrary BffiBs, •mhe»
they are compotmded together-, they tu/// produce fome middle
Thing between thefe two Effeéis. And thefe cannoc be de-
^edto be of good Ufe.

12. Though thefe Rules are for the mofl: part found iz.ThUfre^
to be true, yet it will be very wrong to truft too much ufi,
to them j and I doubt not but the Chymifts themfelves

will difov® them ; fcrr they know very well, that he our makin^
wfio exaítly follows them, will many times form a Judge-
meac contrary to h/xpenence. ^

13. For inflan-^ if we follow thefe tvyo Rules flridl:- 13. The firfi
ly, we muft affirm, that two Bodies which feparately are

coid, oughc together to make one coid Body.

14. We muft affirm, that two H¡yuid Bodies will com- 14. n i„.

pofe one liquid Body. punce.

ly. Thac two tra^ijparent Liquors will compofe one ij.. uj e,.
tranfparent JJuyttor. flanee.

16. That two í'ed Liquors mixed together, will make ¡s. iv r».

one red Liquor. flanee.

17. Thac a Body of a Tellowifh Colour, mixed with a jy. y in~
Body of a Green Colour, ought to compofe a Tellowif} flanee.
Creen.

18. That two things which may be feparately taken is. vi in.
wichout any danger, may alfo be taken together w’ithout /“"“c-
any.

19. However, we know that every one of thefe are con- ,,^,Theflrfi
tradidted by the following Experiments. For Example, Experiment
coid Lime, having coid jVater fprinkled ^upon it, grows
fo hot, as to be ready to burn. Furcher, If Oil of Vi- '
triol and Oil ofTartar, each of which are coid, be mix-
ed together, we Ihall perceive a fudden Ebullición, and
at che fame time a very fenfible Heat.

20. If

ofthe contra-'

112

ROHAULT^s System Partí.

stsi.ii.Expe- 20. If Sfirit ofWine and Sprrit ofUrme, each of which
rimeut. fluida be mixed togecher;, they wilb in a Mo-

meiic almoíb unice inco a Body not ac all fluid;, but pret-
ty hard.

II. m. Ex- 21. If aboutan O unce of Litharge of Silver be put in-

¡mmem. ^ Vmsgar-¡ and boiled half a Quarter

of an Hourj and if a Piece of unfacked Lime be fteeped
Four and twenty Hours in a fufficient Quanticy of Wa-
ter (ic muft be in an Earthen Poc varnilliedj new and
clean,-^ and afcerwards each of thefe Liquorsbe ftrained,
they will be very tranfparentj but when they are mix-
ed, they will become opacous and of a very brown
Colou'r.

^^■Of Sym- 22. In thc Ufe of thefe two Liquors confiíls the whole
jatasuc nk. which they cali Sym^athetick Ink.

They write diac which they would not have feenj wich
the firfi; Water^ and the Writing difappears -the Momenc
that it is dry ; Then, he who receives the Letterj wipes
over the Paper wich a Sponge ever fo littie moiíln^l
wich the ocher Water^ and che Writing begins to
appear of a reddifh Colour, tending to a Black, If
thefe Waters are frefh made^ and Care be testen to co-
ver the Poc in which the unflacked Lime is mfufedj the
Sponge that is moiften’d need not touch che Writing^ in
order to make it appear ; ic is fufl5GÍOT,C3 if ic pafs by ic
ac a liccle diftance : Nay I have ofmn feen the Lime-
Water fo ftrong, that when the Letcer writcen wich
the firít Water was laid upon a Table, and covered
wich a Quice of Paper, the upper Leaf of which only
was moiítned wich che Second Water, che Writing grew
black.

sg. IV. £a- 23. If a Piece of Brafd Wood be boiled in Water over

pcnmut. (.yig p^g¿jj[iy ijjyg 2 Liquor pretty red;

which if ic be afcerwards poured into a Glafs in which
there is ever fo littie Vinegar, chis Colour will be chang-
ed into an Ámber-Colour, and that lo quick, that the
'firft Colour will difappear entirely, as foon as the Water
.ronches the Bottom of the Glafs.

24 . V. Ex- 24. It is certain, that Nut-Galls are of a Yellowilli Co-
¡crimmt. Jour, and that when they are reduced to Powder, there
is no more Blackneís in them, chan in che Copperas, which
is green; and yet if thefe two be infufed in common
Water for a few Days, or if you would have ic quicker,
if the Water be boiled an Hour or two over the Pire, '
they v,/ill be of one black Colour, and not differ from
Ink but only in this^ that they wanc che Gum Arabick.

Chap 21. of Natural Philosophy, 113

25. PhyGcians order fomedmes á few Drops of Sprit VL Ex-
ofNitre or óf OH of Vitriol to be taken in Broth or fome f
ot^r Liquor:. and chefe two Things taken feparately and
in proper Cafes, are good Remedies, buc if chey be ta-
I ken together, they are Poifon. Now this Experimenr,
together with the foregoing ones, and many orhers thác
might have been added, do fo evidently íliow the Un-
certainty of the two forementioned Rules, and confé-
quently the little Uíé of . the Elements ofthe A^itknts and
of the Chymifls, that there is no need of adding any
Thing more : That which ríow remains to be done, is
to endeavour to difcover what afe the true Elémhts of
fiatural ’íh’mgs.

C H A P. XXL

i*

Of the Etements of Natural fhings<.

T HA'^we may a£t here vdth all poííible Gaution, t. That ni
and ®ablifb the number of Elements, upon the in

Confideration of Things as they are in themfelves, with- afmhing fí-
out any regard' tq^e Manner of their aíFeédng us j we
obíerve, that the'nrft Thing that we can conceive to hap-
pen to Matter, is, that it may be divided into a great
Number of Parts, all which areof a certain Figure. This
Confideration is of great Importance, for if we attend e-
ver fo little to it, we fihall be furprixed at fome Perfons,
who are ready to laugh, when we obferve to them, that
the Parts of Matter are of a certain Figure, and yet cam
ferioufly hearken to thofe who tell theip of occult Qua-
lides, which they cannot at all comprehend
2. We obferve further, that befides thofe groís Bodies, 2. That thert
fuch as we can take notice of, with which we are fur-
rounded ^ there are an infinite Number of othersVery imall, •soMet.
which efcape our Sight, and which were not at all known
to the Andents. Though even amongfl: thefe, if we
flriTly examine them, fome may be made appear to us,
fuc-h as the little Eels, which fpring up almoft in a Mo-
inent, in the befl fort of Vinegar fet in the warm Sun ^ but
,it is certain, we had not knówn^of thefe fmall Creatures
to this very Day, were it not for the happy ' Invention
of the Microfcope, in this Age. Thus, for Example, Specks
of Mould upon the Covers of Books, have been long ob-

I fervedj

114

g.T¿/íí thefc
3odies con»
fji of Parts
liill fmalUf»

4 * That Ule-
rnents arife
fram the jirfi
X>l-oijton that
can be of
Maiter,

5 , Thnt toe

do 7ii)t here
fpcak of thc
Divifon that
xoasmade ¿it
the Creatíoti
«/ the World,

ROHAUn^s ?>y%'ízu Partí.

fervedj and alfoj that a Míte, which is much lefs than 3
Grain of Sand, is an AnimaL becaufe we can fee it move
along j buc it is fince the Invention of Microfcope^hat
we can with pleafure fee not only that they are lo, but
that every Speck of Mould is a little Garden covered with
Plants, every one of which has its &talk-¡ Leaves, Buds
and Flowers ; and that a Mite has its Back covered with
Scales, that it has thrce Feet on eacb Side, and two hlack
Spots in the Head, which we luppofe to be Eyes, becaufe if
thePointof a Needlebepucin itsway, it will turn alide.

3. Since fuch ftnall Bodies are difcovered and leen by
the Microfcope, we may reafonably judge that there are
Parts incomparably lefs yet, which efcape all our Senfes,
all che Induíh-y of Man, and exceed even our ímagina-
tion it felf. And that this may be clear by one Exam-
ple j Since a AJite walks along, it muft have Legs, and
thcfe Legs muft neceílarily have Joints. In order comove
the Joints, there muft be Mufles, Nerves and Tendón^
and in chefe Nerves Fibres, fuch as we fee in thofe of lap
ger Animáis, or at leaft, fomeching equivaleñt to them :
And if we would carry this Coníideration yet further, and
fpeak of che Heart, Blood, Braiw, and A'riimahSpirits, we'
íhall be quite at a Lofs, and forced to confeU', that our
Imaginación is unable to comprehend or reprefent the
extreme Smallnefs of the leaft Parts^f which a Mite is
compofed. I delire that theíé Things ’may be well coníi-
dered, and I have purpofely urged them, to avoid thc
Impercinence of thofe Perfons, who ridicule every Thing
propofed to them, which does not agree with their grofs
Nocions 3 and who make a Jeft of it, when we men-
ción that fubtle Matter to them, whofe quick Motion and
Smallnefs makes a PaíTage for it, and finds it a Place
every where.

4. Having laid down thelé Oblérvations, lince we are
aíTured, that the fmalleft Bodies in the World, as well as
the larger, arife from the Mixture of Elements 3 and
lince it is certain, that a fuíScient Number of che fmall-
eft Parts, may compofe as great a Body as we will ,• we
muft conclude, that there ought tobe as many Elements,
as there can be remarkable Differences in the infeníible
Parts of Matter upon their firft Diviíion.

5. Now that my Mind may be the clearer underftood,
Imuft repeat the Advice which Ibefore gave, viz. That
I conlider Things in their mere natural State. And though
I am very well aware, that the firft Diviíion of Matter
was made by God, and as he pleafed, when he created

the

Clíap. 21*' ^ Natural Philosophy; 115

the World j yec chat is not the Divilion I amhere fpeak-
ing ofi. becaufe I believe the Creación to be a Myftery
which I cannoc fearch co the Boctora of. So that I fpeak
of another Divilion^ which may be made agreeably to che
Notions we have, and of which all che Things in the
World are the Confequences.

6. Thus, coníidering as far as I atn able all Macter? 6.whatihat

I firft divide ic in my Mind into an infinite Nutnber of 'r

Parts very near equah not croubling my felf what Figure pofeEkmmts
they areof, becaufe, there may be agreac many other Fi- ta arifefmm.
gures, befides Cubick which comes firft into every one’s
Thought, that may produce the fame Effed:. After chis, I

fuppofe chat God turns every one of thefelittle Particles,
in many difíérenc Manners, about their feveral Centers,'
in order that a true Divifion of chém from each other
may begin to be made.

7. This being fuppofed, it cannot be but that all thefe 7- Tí-’*
Particles of Matcer muft be broken where-ever they are

'Singular, or ase intangled with thoíé that join to them ,• fo Eiemem,
that thofe which were luppofed before to be very fmall,

Imuft becom^ ftill fmaller and fmaller, till they are got
into a S^erical Figure. Thus we have two Sorts of
Matcer d*ermined, which we ought to account the two
firft Elements. And of thefe two we here cali that which
confifts of the vei^ne Duji which comes ofF from thofe
Particles, which áiu not quice fo fmall, when they are turn-
ed round, xhz firft Element . And thefe Particles thus made
round, we cali the Second Element. And becaufe ic may
be, that fome of the fmall Parts of Matcer, either fingly
or United together, may continué in irregular and confufed
Figures, not fo proper for Motion, we take them for che
third Eleinent, and join them to the other two.

8. As .to the chief Properties of thefe three Elertients, 8. The Pro-
ic is to be obíerved, that ic is no Contraditftion to fuppofe

them to be changed from one Sort to another ; Thus the
Particles of the Third Element may fomecimes be made
round, and acquire the Form of the Second. And thoíé
of the Second and Third may be broken, and fo converted
into the Firft. But none of thefe three Elements will bec-
ter preferve their Form than the Second, becaufe ic is more
folid, and the Spherical Figure, which ic is of, will allow
it to move about ic felf, withouC being intangled with the
Particles about it. On the contrary, none are fo eafily
changed as the Firft, becaufe its Particles moving very
quick, and being very fubtie, they cannot refift the Shock
ofthe Particles belonging coche other Elements, when they
I 2 raeet

2i6

ROH AU LT* s Systém Partí.

meet with them» buc are forced at all times to fuit
their Figures to thofe of the Places through wHich they
país, and where their Motion carries them.
t). 7 %í Pro- 9. The Firjl 'Element ought alfo to have more Motion
either of the other Twa, for though all the three
Ekments-, were at the Beginning equally moved by the
Firft Mover, yet it muft afterwards happen, that thsfirfi
Element having oftentimes met with other Bodies which
refiíted it, and which it could not move, will be refledted
back, without lofing any of its own Motion j whereas
the other Elements cannot meet this, but they will move
it, and fo increafe it? Motion by diminilliing their
own.

10. Hbw the 10. And íince the Firfi Élement is often forced to run
mmtacqHircs Intervals which are between the fmall

^reater Velo- Globes of the Second Ekment-, it muft neceílarily be, that
TthJtm’’" Parts being comprcíTed, will leave the Place

where they are, and get forward j and fo having a Moti*
on compounded of their own Motion, and of that oP
the Parts which follow them and prefs upon them, they
, will acquire a greater Velocity than the P^jrts of the Se-
cond Element which forcé them on. In the l^poe man-
ner as the Air contained in a pair of Bellowi'" goes out
with much greater' Velocity than the Sides of the Bel-
lows approach each other, and which by their approach-
ing, puíTi it, and malee it to go out.
ij. vvh¡ai II. Iwould have it obferved by the way, that I might,
próperN^mes Example of ArifiotU, give Ñames to the three

to thefe “eu- forementioned Elements, from the Things which partake
mena. moft of them : Tbus, I might give the Ñame Fire to the
Firji Element, Air to the Second, and Earth to the Third.
But befides, that this would be to a¿l contrary to Order,
becaufe I have not yet proved, that Fire is for the moft
part compofed of the Firjl Elentent, Air of the Second,
and Earth of the Third ; there is yet another Reafon that
ought to hinder me from doing ir, and that is, that I
fhould give Occafion for abufing them, and for having
them underftood in another Senfe than whatlintend they
Ihould be.

12. Perhaps it willbehere faid, that Matterwas not di-
‘vtmtí^arjnot the Bcgioning as I have lüppofed ,• But tho’ I agree

imeginarj. it may be fo, this malees nothing againft me,- for it fig-
nifies very little how Matter was divided at the Beginning ;
and in what manner foever it was divided, there is no
doubt butit is now divided into thofe three Sorts of Matter
which I have deferibed; it beiñg certain, that they necef-

farily

Chap, 21. o/ Natural Philosophy.

íárily follow from the Mocion and' che Dirifion of the
Parts of Matter which Experience- obliges us to acknow-
ledge in the Univerfe. So that the Three Elements which
I have eftablifhed, ought notto be loojced upon as ima-.
ginary Things ,• but on the contrary, as they are very ea»
<y to conceive, and we fee a neceífity of their Exiflenee,
1 we cannot reafonably lay aíide the Ufe of them, in eXf
plaining EíFeÁs purely Material.

I 3 CUAP.

1 ,. IVecánnot reafonably lay ajtde).
Thefe three Elements are to be look-
cd upon as fiílltious and imaginary,
becauíe they depend upon a Plenum
every where, which wc have before
rejefted. Buc concerning the truc
Elements of Nature, the illuñrious
l^exíiton chusexplains himíélf.

It feems probable to me^ that God
in theBe^inningformed Matter in fo~
^ajfy^ hardy impenetrabiet mo-
veable EarticleSi of fuch Sizes and
Fi^ureSs andxoith fncb other PropeY"
ties^ and in fuch Proportion to Spaccy
as mojí conduced to the End for which
heformedijf^i and that thefe Pri-
mithe Parucles being SoHdy are in*
comparably h&rdtr than any porons
Bodies compounded of them ; even
fo very hardj as to wear or

break in Pieces : Nomdinary Power
being able to divide what God him-
felf made one in the jirjl Creation»
(Vhile the Particles continué entire»
they may compofe Tiodies of one and
the fame Nature and Textnre in all
<u4ges : !B«r Jhould they wear away or
break in Pieces^ the Nature of Things
depending on them^ would be changed.
Water andEarth compofedof oíd worn
Particles and Fragmenta oj Particles,
would not be pf the fame Nature ayid
Textnrenoxo^ rpith Water andEarth
compofed of entire Particles in the
Beginnin^. Mnd therefore that Na~
ture may be lajiing^ the Changos of
corporeal Things are to be placed only
in the various Se^arations and new
xAffociatipns and Motions of thefe per-
tnanent Particles j compound 3odies
being apt tp break) not in the midf
of folid ParticleS) but where thofe
Particles are laid iogethery and o»ly
touch in a few Points. Opticks pag-
375* .

Further> nothing c^n be mqre ab-
furd th^n to imagine, that all cheíe
furprizing Things in ihe yniverfe^ a-
rife and were formed out of thofe

three £ieínenfs pf CíirríJa and ’by

the Motion ímprelTed upon them ín
the Beginning, withouc any Inter,
pofitioD afterwards, either of God
•himfelf, or any other incelligenc
Caufe. For according to that Hypo,
tbefis, the Followers of Canes have
not ío much as dared to attempc ex-
plaining how all Kind of Plants ancf
•Mnimal Bodies (which are the prin-
cipal and moñ excellent Part of this
Univerfe) wereat firíf made, and by
what Laws of Motion they were
framed. How much becter does the
foremention'd admirable Períbn exu
prefs himfelf,

Now all material Things feem to
have been compofed of the hard and
folid Particles abovemcntion'd^ vari-
oufy ajfociated in the firfi Creation by
the Council of an intelligeni x^gent,
For ir became him who createá them
to fct them in order, ^And if he did
/o, *tii 7tnphÍlofophkal to feek for any
other Origin of the World) or toprcr.

' tend that it might arife out of 4
Chaos by the mere Laxos of Nature ;
though being once formed) it may
eontinue by thofe Laws for many
ges, For while Carnets move in very
excentrich Orbs in all Manner of Po-
ftionS) biind Fate could never make
all the flanets move one and the fame
, way in Orbs concentrtck) fome incon-
fderable Irregularities exceptad) which
may have rifen from the mutual ^¿ 11 “
ont of Coméis and Planets upan one
anotheTj and will be apt to increafe,
^tillthis Syftem wants a Refoxmatiou.
Sttch a woudeful Unifpr^jity iñ thi
Planetííry Syftem mnft be alloxoed the
Efteü qf Cholee. ,And fo mstft the
Unifor.mity in the Radies ofxAnimals%
they having generally a right and a le ft
Side Jhiipcd alike» and en either Sidi
of their Radies, two Legs behind,
and either two Arms» or two Legst
or two Vi^ings before upon their Shoul-
ders, and bctxoeen their Shoulders a
Neck running down inío a Rack-bone,
and a Heád itpon it j And in tfe

iif

)ti8 RO H AUL'f^s System Partí.

C H A P. XXII.

Qf tbs Farm of a Hará andof a Liquid Body, or
of Hardnefs afid Liquidity, •

iy ” "DÉCAUSE it is by means of our Senfes^ that we
hard and ti- " find out the principal Differences obferved in
quU-Bodies. Thingsj. I tliink we canuoc do better, tban to confuk
them one after another, to find out in what Order the
Forms of natural Bodies oúght to be treated, beginning
with thoíé that difcover to us the feweft Properties of
•their Objeíts. And íince the Senfe of Feeling is the grof-
íéíl: of alh and that which takes up the leal!; Compafs of
our Viewsj I vvill begih my Inquiry with that- Now when
we make ufe of the Senfe of Feeling, to difcover whar
Sort of Bodies they are which furround us, we obferve
that there are fome which refift the Motion of ourHands,
and wili not be divided without great DiíBcult«-; on the
contrary, there are others which do not refift tíetn at all,
but are very eafily divided all ways ,• the firft of thefe we
cali hard Bodies-, and the other liquid ^i>^es ; and we fay,
that a Body is fo much the harder, as títere isgreaterDiffir
culty in dividing it, and another fo much the fofter, as
it refifts lefs, and is divided with greater eafe.' And thofe
Bodies' which are of a middle Sort, betwixt hard and li-
quid, and which refift our Feeling, or the Motion of our
Hand but a little, thefe we cali fifi.

fíead tvpo Ears) tvoo Eyest a Kofe, a
'Mf'itthy and aTongne., aliksJiUiated,
uíljo thé 'jirji Cmirivanct of thofe
"very arirjidaí Parts of ^nima/s, the
EyeSj Earsi Tiram-, Mitfdcs, Hcart^
XatmgSy "Midrifft Qlands.¡ LarynXj
Hands^ Wings^ fvoirnming *Eladdtrss
Tiáutral S^eóiiicleSy 'and other Organs
of Senfe and 'Motion ; and the Inftinfí
ofTirntei ’and ínfeds 'can he the Ef-
fe¿l of nothing e.lfe ih'ari the Wifdum
and Skill' of a' poroerjul^ everlajiing
uígent.^ vaho being in all Places.^ is
more ahle by his W'ili to move the
"Bodies xoiíhjn his boundlefs nniform
Seníbriurrij ánd theréby to forra drid
réformthe Parts ofihe Unherfcy thah
roe are by our V/ill to mave the Parts
ofotir orvnBodies. .And yet roe are not
toeonjídtr the IVorld as the Body of

Gody or theifcveral Parts thereof as
Parts of God. He is an itniform Be--
ing^voidof OrganSy Memhersor Parts.y
and they are his Creatitres fiibordinate
to him^ and fitbfervient to his WilL
^nd he is no more the Sottl ofthem^
than the Sottl of a Man is the Sofíl
of theSipecies ofThings carried through
the Oraans of Senfe» inro the place of
his Senfation^ whereit perceives them
by means> of its immediate prefence
roithant the Intervention of any Third
Thing, The Organs tf Senfe are not
for enahling the Soitl to perceive the
Spccies of Things in its Seníbnumj
hiit only for conveying them thither»
and God has no needof ftith Organs,
he being every where prefent to the
Things tkemfelves, Ibid. p. 378.

3. w§

Chap. 22. ^ N ATURAL Philosophy,

2, We obfervealfo thac a Body, which refifts theTouch
and is wich Difficulty divided;, keeps it felf alfo within its
proper Liroits^ and preferves its Figure, without wanting-
a VeíTd tb contain it; and on the other hand, that a Bor-
dy which does not refiíl the Toucb, does not contain it
I felf within its Liuiits, but runs and fpreads about, if it be'
not put into fome Veílel. Wherefore Ariftotle having
given the Ñame of Dry to a Body which is contained
within its proper Liraits, and that of Moiji, to a Body
which does not do fo, but wants to be contained within
the Limits of another ; it follows that the hard Body we
are fpeaking of is the fame as what Ariflotle called
Dry, -or at leaft a Species of it ; and alfo that the L.i-
\ quid is the fame with the Moijl.y or at leaft a Species
i of it.

I 3. As Ariftotle has not explained what Drynefs and
! Moftnefs coníift in, fo neither has he explained the
ijp^ature of a hard and a liquid Body. But moft of his
Foilowers contend, that a Body is hard, becaufe it com-
prehends a' great deal of Matter in a little Compafs, and
that a Body is liquid, becaufe it contains but a little
Matter i® a great Compafs; fo that they make Hardnefs
to confift in Condenfation, and Liquidnefs in Rare-
faftion.

4. It is to be>?i{erved, that they would be underftood
to fpeak here of a Rarefadion, without the Addicion of
any Matter at all, not fo much as of foreign Matter; and
of a Condenfation which does not in the leaft fuppoíé any
Sort of Matter to come out of the Pores of the conden-
fed Body ; which Things are diredtly oppofite to what has
been before eftablifhed ; wherefore it cannot be thought
ftrange, if we do not agree together as to the Nature of
hard and liquid Bodies.

But if Rarefadlion and Condenfation were made as
they preterid, yet it were eafy to prove that they are mif-
taken in their Notion of Hardnefs and Liquidneís; Foras
the producing one Piece of white Marble, is fufficient to
fhow, that the Nature of Marble does not coníift in
Blacknefs, ib it íhall fuffice to bring one Inftance of a
Body which dilates it felf when it grows hard, in order to
íhow that Hardnefs does not coníift in Condenfation ;
Thus we fee that Water is dilated, when it is turned in-
‘tq Ice* for the Veflels which contained it, and juft

' ■ " I 4 héÁ

119

¿« That hard
and liquid
“Bodies are the
jame kínd of
“Bodies as the
dry and moiji
Bodies of thf
udncUntSf

3, In xohat
the Vollovcert
of Ariftotle,
make Hard>-
nefs and L¿-^
quidnefs fo
confifi.

\

4, That their
Opinión goes
upon a falfe
Síippo/itioni

í". ./í Confíe^
tation of the
Opinión pfehe
Ariftoteliansj
and the Rea^
fon vohy Fef-
fels fíétl of
Water are
broken by íAf

120

RO H AUL'T’s System Partí.

but are many tiipes

tohy Vejfe/s
are broken by
theFroJi,

held it, cannot then contain iCj
broken.

6 . A Mifiake (j, I know vcry welL thac ic will here be anfwered as
teiíans^ flf M ufuaL thac che VeíTels would not be broken^ but for fear
tha Rcajon of ü Vactium : Thac is, becaufe their Sides approach one

anocher, that there may not be any Space left between
their Concave Superficies and che Convex Superficies of
che Water which iscondenfed. But if chis were true, ic
would folio W j that all che Glafs Tubes which we ufed '
in the forementioned ExperimentSj ought alfo to be bro-
keua when no Air got into the Place ouc of which the
Quickfilver carne, which did not come to pafs, as I have
ofcentiraes tried.

7. Anotier 7. Add to this, that if Ice were only condenfed Wa-
í¡mt‘cmdaa-^^'^ ^ to malte for Inílance, a Cubick Foot of Ice, there

muft be more than a Cubick Fooc of Water, and con-

fed Water ^
and vohy it
froims upon
the Water,

fequencly a Piece of Ice would weigh more than a Quan-
tity of Water of the fame Dimenfions. From Whence i,ii
follows, according to whac has been befbre demonílra-
ted, that Ice ought to fink to the Bottom of the Wa-
ter, and not fwim atcheTop, as we find it does.

S. Anociirar 8. But for the full ConvicStion of thofe wbj^' feem to
^nofthlrame Argumcnts, and truñ only to what they fee, lee

TUug. them but take a Glaís of the Shape of an inverced Cone
or Pyramid, and after having filled ic cjfCe full of Water,
expofe it to a greac Froft, thac the Water may become
Icej then if the Glafs holds but half a Pinc, we fhall fee
the Ice rife up about the fixch Pare of an ineh above
the Mouthof che Glaís, which is ^-a Dilatación fenfiblee-
nough not to doubt of the Faft.

9. Whut p. This then is a certain Truth, that every Body which
‘‘ahfrd'sldf ^heomes hard, is not condenfed ; and therefore Hardnefs
cinfjism/ does not confift in Condenfation, ñor coníequently does
Liquidneís confift in Rarefadion ; for as Water is dilated
by freezing, fo is Ice condenfed by thawing. Having
thus fufíiciencly confuted an Opinión which has been fo
long received, and not thinking ic worth while, to íliow
how litde Foundation there is for other Opinions which
have been received only by a few, I come now to efta-
bliflr my own. And firft I examine the Appearances of

í. So great is rhe Forcé of free-
2.ing Warer, thac not only Bowls
and Glafs Cups, but alíb large Vef-
léls of Brafs and Silver are broken
by It. See Exper im, ,yiccad, del Ci?n.
p. yz.

2. A Dilatatíon fenjible enongh)
Yec íc muft noc be diílembled, thac
fomeching may polftbly be here aí-
cribed to the Contra6Hon of the
Giafs. See the Notes on Chap, 23.
Atí, 36.

a har4

121

Chap. 22, of Natural Philosophy.

a hard and of a li<j^uid Body, and find, thac the one con-
tains ic íelf wichin its proper Bounds, and the other does
not : And becaufe to be contaiued wichin its proper
Bounds, is the fame Thing as not to be moved j I con-
clude, that to be hard, is to be compofed of V anides -which
are s fo at refi among themfelves, that their Connexion and
OrdeTí is not difiurhed hy any Matter that moves hetiveen

them.

T. So at refi among themfehef)
Though all hard Bodies have Pares in
^me meafure ac reít, and many
hquid Bodies (vtz* fuch as are made
liquid by Heac) are manifeílly very
much agicaced i yec becauíe fome-
thing more chan che bare Reft of the
Parts feems requíGce to coníiicute
Hardnels 5 (for a Heap of very fmall
Sand» whofe Parcicles are all ac reíl,
is noc a hard Body») and becauíe
M^on does noc íeem always ne-
¿Shry co conftitute a liqüid Body,
(for iome liquid Bodies are very
coldi) I chink ic cherefore worch
while to add fomeching here, co ex-
plainthis Macc^more fuüy.

Firfi then, '^c us hear what che
famousi^fiwíonfays, concerning thac
Forcé by which che primary and na-
turally indivifible Corpufcles of which
ihe Parcicles of all Bodiv-*^^e cora-
pofed, are conneíled and cohere co-
gether.

The Taris of all homogental hard
*Bodies rohich fnlly touch one anotheYi
fiick together very Jlrongly. ^ndfor
explaining hoto this may be, fome have
invented hooked Atoms, which is
the ^Inefiion 5 and others tell
«í, that "Bodies are glued together by
Refíj that is, by an occult ^litality or
Táíher by Nothing j and others that
they fiick together by conlpiring Mo-
tions : 1 hadrather injer from their
CohafoTjp that their Pankles accraft
one another by fome Forcé which tn
immediate Contradi is exiéedh/g Jirong,
at fmall dijianess performs the Chymi-
cal Operations abovementiou* d, and
Tsaches not far from che Particíeswith
any ¡enjible efie¿l— Now if compound
Bodies are fo very hará^ as voe find
fome of them to be-, and yet are very
porotts, and conjifi of Parts which are
only laid together, the Jimple Parti-
(les which are void of Hores, and were
neveryet dividid^ mttjl he mnch har-
dtr, For fnch hfifd Particles being
heapid np togethir, can fcarce touch
one another in more than a few
^ointss and sherefore rduji b& fipsi-.

rabie bymuch lefs Forcé than is re-
quijite to break a folid Particle, whofe
Parts touch m alL the Space betnoeen
them^ without any Pores or Interjii-
ces to weaken their Cohafíon. And
hom fuch very hard Parcicles wbich
are only laid together, and touch only
in a few Points, can fiick together,
and that fo firmly as they do^ with-
ottt the Afjxfiance of fomething wh 'ch
caufes them to be aterafted or prejfed
towards one another, is ve^y ^fp<.uU
to conceive.-~-h¡ow the fmallefi Par~>
ticles of Matter, may coherf by the
ftrongefi Atcraffions, and compoje
bigger Particles of wéakcr Firtue i and
many of thefe may cohere, and com-
pofe 'bigger Particles whofe Virtud is
JitU wesiker : And fo on^ &c. Op-
cicks Ibid. p. 364. 370,

Ic is evident cherefore, that the
Parcicles of which the original and
fmalleíl Parts of Matter are compo-
fed, ftick together and are unicedj
noc by Refi (which is really nothing
at all) buc by mutual Attraciion,
(See the Noces above on Chap. x¡.
Art. If,) And ic is manifeft, thac
all Bodiesj fluid and folia are equally
compoundfd of fuch lorc of Parcicles
encirely íblid and perfeélty hard»
Buc thac which is nexc to be enqui-
red inco, is, whac che Figure and
CompoficioQ of the larger Parcicles
muft be, in order thac che Bodies-
compofed of them, may be hard or
liquid,

Secondly cherefore. Thac Body,
whoíé Particles are ío ficced to each
ocher, as co touch* one another in
large Superficies^, will, by che very
Itrong mutual Atira£TÍun of iis Parts,
be a very hard Body and according
as chofe Parts afterwards either tonch
one anocher only, or are moreover
imangled wich each ocher, will che
Body be more or lefs hrittlc, and ca-
pable of being made liqttid by Heac,
wiih more or leís diniculty : As
íce, VFax, Glafs, Metals, Bones,
VPood, &c.

Thirdlyt

20 . Whatthe
Naiure of a
¿í^tíid 'Body
conjijis in.

RO H AULT’s Sy&tem Partí.

them. Whence it followsj that a Body is ib much the
harder, as it has more Parts which immediately touch each
other without moving.

ló. On the other Hand ; becaufe^ not to contain it
felf within its proper Bounds, is the lame Thing as to
move it felf; and beeanfe we cannot conceive any more
eíFectual Caufe of that Motion which we fee in a li^uii
Body, than the Motion of its infenfible Parts ; I therefore
conclude, 2 that Liq^uidnefi confiñs in the -perpetual Agi-
tatioft of the infenfible Taris of the liquid Body. Thus for
Example, when a Glafs full of Water fet upon a Table
is at reft, though we cannot perceive any fenfible Agi-

Thirdlji That -Body whofe Partí- '
des touch one another in lefs Swper-
iicieSi and therefore are not fo hard^
mayyetbe iriorefoUdi and therefore
Gotd is heavier than a Diamond,
though not fo hard,

Fourthlyy That Body, wbofe Par-
ticlesjwhen they are compreíTed, ap-
proach towavds each other, but do not
Jlip under one another, is an elajiick
Body, and returns to its Figure, by
that Forcé which arifes from the
jnutuai Actrafíion of ics- Parts.

Fijthly, That Body,whofePartIcles
jlip under each other, is a foft Bo-
dy, which yields coche Scroke of a’
Hammer.

Sixthly, Ihac Body, whoíe Par-
ticles touch one another in very fmall
Superficies^ is a crumbling Body, as
S«oi», or luch whoíe Parts may very
eaíily be feparated as' too xoell po-
lifiied MarbleSj which fiíck Cügether
in a Facnjtm, but are puUed afunder
hy che leaíl Shake.

Stventkly, If the Parts of a Body,
either do not touch one another at all,
or at leaíf wilí very eafily Jlip, and are
of íuch a Bígneís, as to be eaíily agita-
red by Heat ; and the Heat be ílifficienr
to agítate them, though perhaps it
be much leís than is required to
keep V/ater from freezingi or if
they be not agítated by Motion, but
are only fmall, round, liippery, of
lucha Figure, and Bignefs, as malee,
them very eafily agitated and give
v/ay ; that is a fluid Body. And
yet.rhc P'articles of fucíi fort ofBo-
dies which are raofi finid, do in
í(;me meafurecohere fogerher,* as is
•e'í’ídent from henee, that ^Iniclfilvcr
very wdl cleared of all Air, will
fland 60 or 70 loches high in the
.Baromecer (as was laid before), And

Water will rife in fmall Tubes opea
at both Ends in a Facuum, Ad(^
Drops of Liquors hangíng upon a
hard Body> and juft ready 10 fall,
will gather themfelvesinco round Fi-
gures in a Vaciinrn-. .viz. by fuch 3
mutual Actradlion of iheir Parciífe)
as that by which the polljhed Mau
bles íiick together, Fiirther,, Tbeíe
fluid Bodies, if they have Particles
which can eafily be intangled witli
one anofher, as Oil(''iX fuch as may
be raade fliff by Colds and faftned
together, as if they had Wedgesput
berween them, as Water, fuch Bodies
eafily gr-^i^^hard. But if they have
fuch íbrt of’ Particles, as can neiiher
be intangled with each other, as Air,
ñor made ílifF by Coid, as §lfiiá<
filver, íhen they cannot by any meant
be madetocongeal.

Eightbly, If che Parts of a Body
be very fmall, ípherical, and exceed*
in| denfe, fuch a Body may alfo be
finid, and yet be much heavier, than
hárder Bodies, whofe Particles are
ñor fo fulid, but which touch one
another inhrger Superficies*

Ninthly, Thofe Bodies,whofe Par-
ticies ‘are agitated wirh a very quick
Motion all ways, whatever the Fi-
gure of them be, will be liquid, as
Metals that are mclicd, &c. Bus
fuch Bodies grow hard, as foon as
that víolent Motion ceafes,

Lafilyj Thofe Bodies, fome oi
whofe Particles are intangled w¡{ii
each other, íome ol which touch one
another in lange Superficie.^, and fome
areloofe, and will eafily ílip under
each other, thefe are fiexile as Lea-
ther, or very pliant as Twigs, Gltiti
Pitch^ &c;

2. That Li(jHÍdncfs confifis) Sec
the ^o.tes on ^he foregoing Artic.

taaoil

123

jChap. 22. c/ N A T U R A L P H I L o S o P H Y.

tation in it, yet iptwithñanding, fome of its Parts are in
Motion downwa'rdsj and at the fame time others of
them are in Motion upwards, fome of them move from
the Right to the Lefr, and others from the Left to the
Rightj in a Word, diere are fome Parts of the Water
which move in all manner of Determinations ,• whence
je follows, that That Body is the moíl liquid, whofe
infenfible Parts are the fmalleft, and the moíl agi-
tated.

11. If what I have now íaid of Liquidneís be joined

to v/hat was before faid concerning Hardnefsj we ñiaW fift^adyem-
eafily conceive that a foft Boáy, which íeems to be of afP"'-
middle Nature betwixt a hard and a liquid Body, and to
partake of them both, is therefote foft, becaufe it is com-
pofed oftwo Sorts of Parts, the one in fome meafure ai
refi, and conneded with each other, while the other are
?■» Motion, and thereby caufe fome fmall Agitation in the
íSmer.

12. Now that which confirms me in my Opinión con- “

cerning the Nature of hard and Uyuid Bodies, is, that the ,ef,¡is the
chief Proj^rties of them are neceíTarily deduced frem Tmah.
thence. irod Firíl, Suppofe the Nature of a hard Boay

to conGíl in what I have faid, it follows from thence,
that it mufl be laitl^iffculty divided : For, for Inñance,
if I put my Fin*r to any of its Parts, I ought to
íeel the Refiílance, not oniy of thofe Parts which
I touch, but alio of all thofe Parts which are behind
them j and many times it is much eafier to move the
whole hard Body, than to feparate one Part from it, be-
caufe the reíl of the Body has a ílronger Connedion
with, and is more at reíl, with refped to this Part, than
the neighboUring Bodies have with the whole Body.

13. On the contrary, fuppofe the Nature of a liquid
Body to confiíl in what I have faid, it follows from thence,
that a Liquid muft he very eafily divided. And indeed if did.

I put my Finger to it any way, it meets with no Re-
íiftance,' for thofe few infenfible Parts which my Finger
touch, being in Motion already, are very ready to quit
jtheir Place j neither are they fupported, or hindred

¡ly the Refiflance of thofe which are beyond, which ara
slfo in continual Motion, and therefore eafily yield to
bem, and open a Paííagc for them all ways.

14. What I have advanced concerning the Nature of

3 hard and of a liquid Body, is ílill further confirmed from preJcttedZt
henee, that all the Confequences that can be drawn from mrapted,

|t, help tq explain fome Experiment, which perhaps it

WOUld hard "Btdy,

124

ROHAUL^T’s System Partí

would be itnpoffible to explaín withom: it. And fírfl, ií
we confider that fotne Bodies are eafily- alterad, only by
difturbing the Order of their Parts, and that every Thing
endeavours as much as it can to continué in that State iri
which it is, and confequently that which is once at reft,
will never begin of it lélf to move ; it will not be diíE-
cult to find out a very eafy way to preferve a hard Bodj
a njery long time, viz. hy inclojing it m another hard Body ;
whofe Parts being at reíl among themfelves, can mak?
no ImpreíEon upon it, and are moreover a Guard upon
it, againft the Aílault of any external Caufes which might
tend to corrupt it. And thuswe feethat Salí, Sugar,and
Metals, are preferved by being thus inclofed in hard
Bodies.

ly. ofthe 15. On the other hand, it is eaíy to forefee, that the
rr ought to happen, if hard B$d‘es be put into Li-

fihe llrtaiñ • For the Parts of Liquors being 1 in continual Agi-
2oá?M. tarion, they may eaíily ío Jhake and move the Parts of hí||j|
Bodies, as to forcé them out of their Places, and carry
them along with them. And thus we find it by Experience,
in all hard. Bodies that'Can be altered, as in Sugar and
Saks, which are diffipated and fink to theBíf^rom ofthe
Water almoftina Moment ; iníbmuch, thatif we throw
a Pound of Sugar into a great Tub of Water, it will in-
tirely difappear in a íhort time j and Parts of ir, ^ wiO
alfo be fo diffipated, and fpread amongft all the Drqps of
Water, that there will not he one of them bqt what is
impregnated with ir,

Bodies may be compofed of Parts
noTZntirdy of dijferciit Bignejfes, as well as liquid Bodies, it is eafy to
¿jffdve cer- conccivc, that therc may be fuch a Liquor as will car-

tam “Bodies*

I, In continual jdgitation) Seo
the Noces upon Are. 9.

2,. Will aífo be difjipated) The il-
kílríous i'TíiPfo» Chus expreíTes him-
felf upon chis Subjeft in his Opejeks,
p. 362. If a very frnall ^antity of
any Sale or í'^triol be d’Jfolved in a
great ^innntity of Water \ the Pár-
eteles of the Salt or Viiriol xoill not
Jinh to the Bottomt thostgh they be
heavier- in Specie than the Water ,
hit will cvenly áifjttfe themfelves in-
to all the Water, fo as to make it
as f aliñe at the Top |íf at the Bot-
tom. And does not this imply, that
the Parts of the Sale or Vitriol re'
eedefrom one another, and endeavonr
(o expand íhenfelves, Aytd ¿et as far

afisnder, as the §ÍKa 7 ttity of Water
inxohichtheyfioaty willallovo? ^And
docs not this Endeavonr i*nply, that
they have a repnljtve Forcé by xohkh
theyfly from one another, or at leajl,
that they attra 6 } the Water (See
che Noces on Chap, xi) more (Irongiy
than they do one another? For all
Things afeend in Water^ vohich are
lejs attraÜed than Water by the
gravitacin^ Povoer of the Earth;
Jo all the Partióles of Salt which jloat
I in Water, and are lefs attraded
I than Water by any one Partióle of
I Salt, mnji rcccde from that Payticle,
; and give way to the more aítraíled

125

ChapV22. of Natural Philosophy.

ry amay '¡vith it only fome certain Varis of a hard Body,
and that otbers "wiUsiot hedijplaced hyit. Thus Water will
only waíh oíF the fineft Parts of Liquofiíh, and leave che
groíTer ones at reft with each ocher.

17. Ic may alfo ib happen in hardBodks, that theVarts yj.of the

of them which are pretty near equal, may yet be fo folid j ^

and on the contrary, all the Parts of a certain Licuor gua fottis.
may be fo fmalh that the Parts of the hard Body wiU

not be at all moved by them, as they v/ould bb by tne
gropr Parts of another Liquor ; which doubtlefs is thé
Reaíbn why common Water, will not diíTolve Sllver, and
why Aqua Fertis, which the Chymijfs czll Spirit of Nitre • *
will eafily diflblve rVj buC istoo weaktodiflblveGold.

18. However, ic is not only the Grofsnefs ofthe Parts 18. virhj
of any Liquid, which renders ic capable of feparating the

Parts of a hard Body j the Pores which are between the jihe ’‘°s¡her.

Parts of a hard Body, do alfo contribute towards ic : ’ For

they may be of fuqh a Figure, and alfo fo fmall, that the

rSts of the Liquid cannot penétrate them j from whence

we may conclude, that the Parts of the Sales of which

Afta regia is made, are put together in fuch a manner,

as to com^fe Bodies ^ too grols to enter the Pores of

Silver, andTo only íliding by them, they can neither go

in, ñor divide the Parts ; Wherefore it is not to be won-

dred at, if chis Wamr will not diffolve Silver, though ic

dilTolves Gold.

15. Ic

í. WiU eafily dijfohe it) Con-
cerning the diílolving of Metals che
fame celebrated Períbn fays thus.
Wiien Aqna Fortis, or Spirit of
Vitriol potired upon Filings of Iran^
dijfohes the Filings voith a great
Heat and Ebttllitiony is not this Heat
and Ebullition efe¿led by a violent
Motion of the Parts, and does 7 iot
that Motion argüe, that the acid
Parts ofthe Liquor rtiJJj towards the
Parts of the Metal with violente, and
rmforcibly into its Pores, tiU they
get between its ntmofi Particles, and
the mam Mafs of the Metal» and
furrounding thófe Particles., loofen
them from the main Mafs and fet
them at libcrty to float o^ into the
yy nter ? And when the deid Par-
tkles, which alone would diflil voith
an eajy Heat, will not feparate from
the Partéeles of the Metals, xoithont
avery violent Hf ata does not ihiscon-
frm^ the AttraClion between them ?
Opticks, p. 35-2. Now this fame

Aqua fortis which eafily diflblves
Iron or ^ilvera will noc diíTolve Gold
at alI ; the Reafon of which is, be-
cauíé its Parciclesj which are more
ftroDgly attrañed by the Particles of
Iron or Silver rhan by one another,
are on the other hand more ílrongly
attraíied by one another than by che
Particles of Gold. The contrary fo
which we are iO’ underfland ot that
Forcé by which Gtld is diílblved in
Aqua regia.

2. Too grofs to eníei) Mr,' Clerc
in his Phyjicks, "Baok il. Chap. iv.
Scél, 24. contends on che contrary»
that the Parts of Aqua regia, are
fharper and fmaller than thofe of
Aqua fortis, and therefore can enter
the very fmall Pores of Gold onty» and
feparate its Parts, which lite Vyedges
they drive from one another, wkilji
the groffer ones move about the Sti-
perjicies of the Gold to no purpuje,
they not heing able to dijfohe the co/i-
tinnlty ofita becaufe they cannot eu”
ter

P

%26

19, The Me-
thod of fcpa-
rating Gold
from Siher»

2,o^

£aTts 0 / ma-
T,y “Bodies
mhích are
heauier than
Vf^aieTj da
Tiot Jtnk in it.

RO H AU L’T's Sy%tzu l*artí,

19. Ic is from the Confideration of the diíFerent Pro-
perties of the feveral Sorts of Aqua foríis, thac the Refi-
ners of Gold have lately found out a way of feparating
Gold from Siher mixed with it : The whole Secret of which
confifts in putting the Maís compofed of Gold and Sil-
nier into Ai]^ua fortis, which will diílblve the Siher only ;
for then ics Parts will be brought out by thofe of the
Liquidj till the puré Gold will remain lilje Sánd cr Dregs
at the'Bottomof theVeíTel; fo that by inclining itgent-
ly^ and pouring the Aqua fortis into another Veflel, it
will carry the Siher along with it, and leave the Goli
ac the Bottom ; After this, they feparate the Siher from
the Aqua fortis in the following raanner j they put j
Quantity of common Water to the A^ua fortis, to make
it lefs corrofive, and then put in a Pieceof Copper, againll;
which the Particles of Siher brought out by the Liquid
ftrikíng, they are ftopped by itj in the íame manner as
Duft flying about a Room is ftopped by the Hangitws
or any other Furniture which is foft, or as a Stone ftidÉ,
when it is caft into a Mortar. The Gold and the Siher
being thus feparated from one another in Duft, may each
of them be melced in a Crucible, and then made diftinfl
MaíTes of.

20. Ic may here be asked, why the fmall Partióles of
Sales and Metals, fwim thus in ail the Parts of common
Water or Aqu a fortis indifferently, dG/ whence it is, that
they do not iink to the Bottom of the VeíTels.? For this

terits Tores. And agaÍHj Señ. 28*
He faysj That from the Mixture oj
•many Saks, the París of the Aqua
regía bec^e fmallert and more ftted
to enterthe fmaltejl Pores-, and fepa-
rate the {hiallcfl Parts \ hetxoecn vohich
they are driven Uke l^'edgeSi by the
Motion of the Liauid in vohich they
fvoimi but xohen they enter into roider
PoreSy they have no Efeél > in the
fame manner as the Forcé of Wedges
to feparate Things joined together, is
nothing imlefs they be driven into
Jireight FiJJnres. S¡nce therefore the
Pores of Gold are the fmallefi oj a.
ny Metaij they will admit the Par-
ticles o/Aqua regia 07 /^j and thegrof-
fer Par/s qf Aquz tonissannot cnter
into them, idoxo the fame Parts of
Aqua regia are too fubtle to have
.Strength enottgh to remove the Sides
of the Pares of other Metals '3 for
they veant the grojfer Parts of Aqua
fortis which filland divide the larger
Pores* Thusfarbei 6uc whac he

íays, he does not confirm by any
Arguments or Reafons, unlefs it be
this, thacSilver feems to have larger
Pores [han Gold, becaufé ic is lighter j
but from the known Propercies of
Silver, its hardnefs, fnioothnefsj &c.
we may with much greacer Proba*
bilicy colleft, that ic confifts of fmal-
1 er Particles, and therefore has fmal-
ler Pores, though more ot them i
But thac Gold on che con-
trary, confifts of * lar- * See Part
ger Particles orLumps, III, Chap.
and ío has larger Pores, vi. Are, 151
but much fewer. And as
to the Nacure of the Liquids, I
íhouid think, thac the Parts of the
regia i would become noc
fmaller, but larger by che Mixture
ofmany Sales. Butall thisdepends,
as was faid before, no: fo much up-
on the Bigneís and Figure of the
Pores, as upon the diíFerenc Atcraéb-
on of the Parts.

■ fliould

€hap. 22. ^Natural PHitosopHt." Í27

íhould íeem to follow firotn what was before demonñra-
ted concerning hard Bodies fwimming ia Liquids, becaufe
every Partióle of Salt or Metal is heavier than an equal
Mafs of the Liquid in which it fwims. However, it is
to be obferved, that when we reafoned in that manner,
we coníidered only the Gravity of the hard Body^ and the
eafmefs ofthe TJ^nid to be dividel-, we didnotthen know
of the 1 Motion of the Pariicles of the Liquid, by which
they carry up with them as many Partióles of Sale or Me-
tal, as would defeend by their otivn Weight j in the fame
manner as the Bubbling up of new Wine, makes other
Bodies which are heavier, fwicn, and not fink to the Bot-
tom of the Tub ,• where we fee that they do at laft fub-
lide and compofe the Lees, when this Motion, which is
greater than the ordinary Motion of the Liquid, ceafes.

To which may be added, that the Partióles of the dif-
folved Body are in fome meafure intangled with thofe of
the Liquid, which they go along with ,• which íhows us
líbre particularly that this hinders them from being able
to fink.

21. And that which is remarkable here, is, that as the ar. T%at
Partióles the Liquid are finiré, and the Forcé by

which they are agitated is limited; it tnuft neceíTarily fol- yvlúlmiu
low, that when they have once laid hold of as many Par- d¡u'ohe mfy
rieles as they can jj'Mtain, they cannot after that feparate
•any more, ñor ovemome the Refiftance of the remaining tird ■Bodj.
Partióles which are at reít i wherefore the hard Body will
be no farther diíTolved. And thus we find by Experi-
ence, in common Water and Aquafortis, that they will
diíTolve but a certain determinare Quantity of Salts or
Metals. Thus, for Example, if, after a Pint of common
Water has diíTolved a certain Quantity of Salt,, one Grain
only be put in, it will continué whole in the Water, as
it would do in a dry Place.

22. And from henee it follows, that if after a Liquid 22. a» «fe
has feparated all that it can from a hard Body, it be eva-

porated to a certain Quantity j that which remains will °m^¡ ]¡
not be able to contain all the Particles of the diíTolved
Body, wherefore many of them will be forced to unite
together, and to compofe íbmething fenfible ; And thus
it is, that if Water be boiled, having firfb been ftrain-
ed like Lye, through Earth charged with Nitre as
much as it can be, and then taken off from the Fire,

The Motion 0/ Tarticles) Not by- 1 See abPvc on >Art * i jr.
íte Motiüfljbuc by their ActTaf 5 lioii, l

and

í 28

25. That the-
yyater
which Toill not
dijfolve one
ctrtMn Hody
any /ow^crj
'SoULyet id'f-
foive a Body
of anothtr
Sart.

24.. fíom tht
'S'ff.ipitatim
of the Chy-
‘mijisismade^

U.OU AULT* s System Partí

and permitced to íéctle a lítele, a great many Particles o!
the Salc-pecer which are diíingaged from the Particles oí
the Water, will ceafe to move, and ílriking many oí
them together againfl: the Concave Sides of the VeíTel,
will at lafl: compofe ' thofe curious Bodies in the Forni
of Hexagons, which we fee ílick diere. And in the fame
manner we may apprehend, how all the other Chrifid-
zations of the Chymifts are made.

23. Thongh a certain ^antity of any luiquid, will dif-i

folve but a dterminate ^¡uantity of a certain hard Bojy,
yet this does not binder, but that other hard Bodies may
be diffolved by the fame Liquid ,• becaufe their Particles
may be of fuch a Figure, as to fuit with the Particles of
the Body already diíTolved, in fuch a manner, as may oc-
caíion more diffimilar Particles, to move with greatet
Eafe, than the fimilar Ones could move And thus Ex-
perience íliows us, that after Water has diíTolved as much
Salt as it can, it will yet diíTolve a fmall Quantity of Vi-
triol or Alum. (Ji

24. If a Body be put into a Liquor, to whofe Parti-
cles it will more eafily unite it felf, than to thofe of ano-
ther Body which it had before diíTolved ; and fimpohng alfo
that it cannot comprehend thefe two Sorts 'bf Particles
together, ^ it muít be forced to let go the Particles which
it had before embraced, which will confequently fubfide
to the Bottom of the VeíTel. ThusC‘'a littlc of that díf-,
folved Salt, which Chymifts cali Oi/ ofTartar, be poured
upon Aqua fortis which before had diíTolved Silver, the
Metal will be forced to fubfide to the Bottom óf the
VeíTel. And this Inftance fliows us theReafon of all the
Trecipitates of the Chymifis.

T. Thofe curiotts "Bodies) Concern-
íng which the admirable Perfon be-
fóte citsdjíays thus. Whenanyfaline
Liquoris evaporated to a Cuticle) and
let coolt the Salt contretes into regular
Figures; which argües ^ that the Far-
ticles of the Salt before they concreted^
fioatedin the Licjaor at cqttal Difian-
ces in Rank and File» and by con.
fsquence, that they aóied upon one a-
nother by fome Power, which at equal
Dijiances is equal, at unequal Difian-
tes imcqnaU For by fuch a Fovoer
they will ranga themfehes imiformlyi
and withotit it, they will jioat irre-,
gularly, and come togethtr irregular-
ly. Opiicks, p. 563,

2. Tt mufi be forced to let ^0) If
füch a Body be put into fuch a Son
of Liquor, that the Particles of the
Liquor will be more ftrongly ai-
traéted by the Particles of this Bo-
dy, than by che Particles of that Bo-
dy which was diflblved in it beforci
che Particles of che Liquor being by
chis ílronger Aterañion removed
from the &rft Body to chis Other,
will fuíFer che Parcicles of che firft
Body to fink to' che- Boctoma in che
fame manner as Iron is feparated
from a Loadftone, by puteing a
ílronger Loadílone to ic.

■ 25;. We

Chap. 22. of Natural Philosophy.

129

35', We muft not here omic anocher Circumftance ve- Ho-tufao
ry conííderablej and that isj thac rhe Particles of tvio L.i- -

qtioTs may be of iuch a Bignefs and Figure, as to intangle mayjomplfi
one anocher when they meec together, and fo move with Bü-

more difficuity j whence it follows, thac they will com-
pofe Oks Body luhich is 7iot fo liquid : So likewiíé, if che
Particles of the two Liquors adjufl: themfelves to each
ocher, fo that the greateft Pare of chem are hindred from
moving, rhen all the Particles together will form a Body
fretty bard. Thus we íce, that if an equal Quantity of
Spirits of Wine and Spirits of Uriñe, each of which Li-
quors are very fluid, be mixed together, they will unite
into a ¡retty bard Body.

26. We may add to whac has been íaid about the Mix- jj¡,„ ^
ture of differenc Liquors, thac there may be found o?ie, which >>“'•‘1 "Body
is compofed of fuch fort of Particles, that fome of them

being much larger than ochers, they cannot continué their om oniy.

Motion, but by means of the fmaller ones j fo that if

mefe be any way difingaged, the Weight of the other

alone, or the Irregularity of their Figure, will make them

continué ac refl: with each other, and according as they

are more 'J|- lefsclofely united together, they will compoíea

Body more or lefs bard : And this is the Reafon why fome

of the Particles of Milk or Blood curdle, while others

which are more put^er to cpntinue their Motion, being

difingaged from theíe, compofe a Serum, wliich remains

liquid. And this is alfo the Reafon why, in fubterraneous

Caves, which they cali ároppirtg Caves, certain liquid Drops

which diftill from the Roofs barden into Scone, after they

have been a lítele while in the open Air. ,

27. Having fufficiently flhown by chefe Experiments, 2.7- Ofthe
thac the Particles of liquid Bodies are in continual Agita-

tion, we are to enquire nexc, what the efficieiit Caufe of
this Motion is, firft, in Water and other fuch like Li-
quida, which feldom grow hard, but more particularly
in Air, which never hardens, but always remains li-
quid. Wherefore in the firfl: Place ic is reaíonable to
think, that che f Figures of the Particles of Liquids are t For if
not altered, fo long as we cannot perceive any kind of
Alteración in them : Buc further, becaufe they cannot move nuaiiy aiter-
withregard to each other, as they ought to do, to com-
pofe a Liquid, without leaving a great many Interftices ^e°d of fubm
round themj which there being no Reafon to think Matiertofiil
empey, they muíl neceíTarily be furrounded by fome Mac-

330 ROH AULT’s System Partí,

ter which is very fubtlcj fuch as chat which we before
called the Firft or Second Element. And as the Particles
of hard Bodies diíTolved in any Liquid;, are kepc in Mo-
tion by the Particles of this Liquid j fo we ought to think)
thac the Particles of Watei% and of all Bodies which do
not congeal, but always retnain liquida are in perpetual
Agitation, becauíé they fwim in the Matter ofthe Firji and
Seco?i¿ E/emení.

aS. Hoa Li- 28. If this Matter be very much agitated, it is eafy to
Val at^d it may^move the Particles of the Liquid

in fuch a manner, as *to diffipate them from each other,
and make thetn fly into the Air, and this is called E'va-
poration.

ag. Howthey sp. On the other hand, .if its Motion be very fainü, or
arccmgeeied. if ¡t be more than ordinarily fubtil, it will follow, that
it will not be capable of preferving the Liquidnels of fome
groíler Bodies j in the fame manner as we fee the Water
running amongft Bulrufhes, keeps them in Motion, and
diílinít from each other, whereas in the Air, they are
confufed and mixed together, without any Motion ; i and
thus the Water isj 9 »ze» in Winter, and turned into Ice.
But we cannot flrow a Reafon why this hapjpins at one
Time of the Year, rather than at another, tríl we come
to know fomething more of the Syftem of the World.
^o.Whyfome 30. If the DifpoGtion of the Par^.les of a Body be
fofthcffre^ fuch, as to leave Pores between thetn íarge enough to re-
they bicme ^cive the groíTer Matter of the Firft and Second Elements,
liquid. this Matter may íhake the Particles a little, before it quite

feparates them, and moves them from each other, and
confequently the Body ought to grow faft, before it hecomes
liquid, as we fee Wax does.

31. m^o- 31- But if the Pores of a hard Body are fo fmall, that
ther 'Bodies _ Qoly the moft fubtil Matter of all can país through them,
‘’mi'thmt'Vrow- Cafe, that which is more grofs, and which is alone

iag fijt? able to íhake thofe Particles which make the leaft Re-
liftance to it, can only apply it felf to the Superficies of
the Body t whence it follows, that it will have diñolved

I. j^ndthj's theWater) Sincenei-
ther the Forcé it freezes wirha is al-
■»^’ays proporcionad to the Co/d, but
íeems to have fome Dependance op-
on other Changes in the Heavenss
ñor is the Coid, unleísfo far’as it is
merely comparaúve [See the Notes
o« Cfjdp,^ xxiu, xArt. owing. to
the Parcldes. being at Reft 5 ñor
can fiardneís it felt (ó'íe the Notes on

Art, 9. oj this Chaj>.) arife from th«
mere Reft of rhe Particles : Con-
gealing muíl neceíTarily be.aícribed
either to nitroiis Particles; or co the
Particles of íome other bales, which
like Wedges fixcd between the Par-
cicles of Water, join them together
and make them cohere : However
there is hitherto nothing cercaic found ‘
ouc concerning chefe Particles.

all

Chap. 22, o/ N ATURAL Philosophy, 131

all the esternal Parts of the Bodyj before it makes any
Alteration within it. And fo fuch a Body ivilí be entire-
ly dijjblved •without being made foft, as we find Ice
does. *

32. Ic is not at all furprizing, tbat Water^ which is li- 3^-
quid, fhould foften a great tnany hard Bodies which it pe-
netrates and diíTolves, and that, when k is mixed with ofFarh.
Plaifter of Paris, for Example, there fliould ariíe a Com-
pofition pretty liquid ; But ic is very furprizing, that af-
terwards ic .fliould atquire a Hardncfs which it would ne-
ver have had without mixing Water wich it, which one
would chink, fliould rather help to íbfcen, than to barden ic.

Ñor can we chink, that chis arifes frocn a fudden Evapo-
ración of the Parts of the Water ; for if it be weighed
when ic is liquid, and weighed again whenic is grownhard,
wecannotperceivethat ic has loílanyof its Weight. My
Opinión conceniing the Macrer is thisj that the Pire has
■Bmied a great tnany Pores in the Plaifter, of fuch a Big-
nefs, as che groffer Particles of the Air cannot penetrare,
becaufe they are not folid enough to remove the Obíla-
cles they.^eet with, which the Particles of the Water,
which aré^more folid and penetrating, are able to do.
Wherefoi^, when the Plaifter is moiftned with, or puc
into fuch a Quantity of Water only, as is fuíBcient to
furround every Gf'i^ or Lump of ic,- and afcer that they
come to be ftirred up together, then the Particles of the
Water which forcé chemfelves into the Pores, like fo
many fmall Wedges opening and fplitting them, divide
thefe Grains into ftill fmaller Pareéis. And becaufe thefe
Pareéis have a larger Surface than the Grains had before,
of which they are but the Duft, it is more than the Wa-
ter is able to furround. Infomuch, that the greaceft pare
of them touching one another clofe, and continuing at
reft, ic is no wonder i if they compofe a hard Body.

K 2 33. From

* The tme'Caufe why Tome Bo-
dies grow foft before rhey melt» and
others notj feems co be this ; chac
thofe Bodies which grow íofcj are
compofed of dillimilar PartSa fome
of which melc íooner chan thofe they
are mixed wich.

I. ]f they compofe a hard "Bcdy)
Mr. Le Cltrc attacks our Author here
with three Argumenesin his Phyjichy
%ookV, Chap, Xiv. SeCf. zy. Firji,
fays he, Th¡s Anfver does not agree
with a Mafs made up of Meal and
’PVatcr kneaded together i and baked »

and Qther fuch like Thmgs that might
be bifianced in, Buc can any Thing
be more evident, rhan chac the Eva-
poración of che water produces che,
fame Etfe6t in Bread, as che Difíülu-
tions of the Lump-, in Plaiíter of
Paris*? For chough not all, yec cer-
taínly Torpe of the Water is difl'ol-
ved into Vapoursa in proporción co
che Heac, wherefore che external
Paje of the Bread is much harder chan
che Incernal. Secondlyi Helays. He
does not fhüw rohy the Particles ofFk’dm
ter fo dividid touch one arnther chfe.
Bu;

j¡2 RO H AU LT’ í System Partí,

■¡i.Thattao From lieDce we draw this Confequence, that if

Plaifter be put into fuch a Qaan'ity of Water as is
ivater hin- fuíBcienc to furroLuid all the fmall Pareéis which the
ainthiPiai- Lumps are divided into, they vvill be hindred from reft-
grminlhari. íng) and fo the Plaifter -will not grow bard at allj and
Chus the Mafons find ’ic by Experience, and this is
vvhat they mean, when they fay their Plaiíter is
drowned.

■34. why Notwiuhftanding this, itisnotto bewondered at,if

mTharfea, Bodies which the W ater will divide, and yet

Lime. ' not at all help to unite and barden their Parts into one
Mafs, as it dogs thoíe of Plaifter of París j for the Par-
ticles of thefe Bodies may be of fuch a Figure, as fcarce
tú touch one another at all, and fo cannot unite together
to compofe one Whole : To which it may be further ad»
ded, that the Water has fo qtiick a Motion within fome
■ Bodies, that it difperfes very much the difuniced Par-
ticlesj and by this means the Pores and Intervals, whic^J
are between them, becomé fo large, that the Air has Pow-
er to get in, and hinder fuch Particles from touching one
another. And this is the Reafon why Lime, which is di-
vided by Water, does not however beceme hardCke Plaifter.
of París ; For if a Piece of Lime, which has been wet-
• ted with a little Water, be divided without medclling with

\

But he does expreíTIy fliow chis in
thefeWords. ^ndbctau[e thcfc Par-
tlcUs have a larger Surf/ice than thn
Grains haá before, of xohlch íhey are
but the DuJ?, it is more than the
Water ¡s able to fnrroimd i Info-
vjueb that the greateji Tart of them
tonching one another chfe, &c. Whac
coiiJd have been íáid more exprefs ?
Buc (I fuppofe) chis learned Gentle-
pan, when he tranílaced this Place
into Latid-, being not very a£tenrive>
overlook’d che conne61:ive Parcicle,
tclUment qrie» Thirdly, He /áysj
That he Jhppofes Hardncfs to arjfe
from immediats Contaü and
which ixe have before confrited. Con*
cerning chisj Ses the Notes, on xArt»
the <yth of this Chap. HaWng chns
cojifuced the Opinión of our Anchor,
the learned Gentleman conjetTcuresa
That the Particles of Water which
‘‘ dijfolve the grojfcr Lumps of the
Plaijier.^ are fo fixed into the lef-
Particles, as-, libe JVedges to
‘‘ joh many of them together^ and fo
a more folid Mafs. Butj

íf the Parts of che Plaifter muft be
kept cogetber by Wedges, it íéems
much more probable, that che burnc
Parts (for the Plaifter is made of
Stone half burnc) growing a lítele
hot, by the Water being poured on
icj draw che voladle Sales ouc of the
Parts wl¿ch are not burntj which
Particles of che Saks being fixed in
che Pores of the Plaifter, keep ¡es
Parts together : For the ftiff Particles
of Sak, feem much more proper to
perform the Office of W edgesj than
che Umber and flexible Parts of Wa-
ter. Buc indeed, Plaifter of Paris,
Clay, and fuch kind of Bodies, do
cherefore growhard in this manner,
becaufeche Water in evaporacing, fo
accradfs their Parts to each ocher,
which before didnoc touch oneano-
cher, chac afterwards touching one
another in larger Superficies, they
cohere together by tba: mutual Atcra-
¿tion, v/hich depends upon imme-
diate Concaff. See the Notes on Art,
the ^th of this Chap,

Chap. 22. o/ N ATURAL Philosophy, 133

ít, the Duít into which ic diffolves it feifi, is of two or
three times as much Bulk as ic was befóte.

3^. When the Water penetratcs the Potes of certaiti
Bodies which it cannot entirely divide 3 it isevident, that Fhfimísl-
ic will ftop fot fome time 3 becaufe it muft lofe its Mo- condEUment
don, by ftriking againft the Parcicles which it touches :

Butic is otherwiíé with the Matter of the Firjl and Seoond o/hardEa-
Ekment, when it paíTes chrough the Potes of hard Bo- *'"•
dies : Fot as thefe Potes, as fmall as they are, are formed
by its concinual paíling through them, ib ic leaves them
big enough always to find a Paffage through, wichout ever
being ftopped.

36. Howevetj it is to be obferved, that by bending a

hard Body, fuch, fot Example, as the Blade of a Sword, ofthFuatter
the Partióles will be made to expand themfelves on the f the Semid
ConvexSide, and to contradi; themfelves on the Concave fjf^thrlugh'
Side, fo that its Pores will become fmaller and ílreight- very fmaii
^r on this Sidej buc chis ought not to hinder the Matter
*f the Firñ or Second Element from entring in, becaufe °
being very fine, and moving very quick, ic ought racher
to alter its own Figure and b^ecome longer, br to wear in
pieces th 1 | Matter which ílreightens it, than to be hin-
dred in its PaíTage 3 and fo the Pores will not be ftop-
ped up by it.

37. But becauSyhe fubtil Matter which palles through

the Pores which are fo very fmall, cannot endeavour to ¡¡ringing
wear the Partióles of the hard Body through which ic bacfconjifis
paífes, but it muft at the fame time endeavour to reftore ■
tl;e fame Partióles to the State they were in before the
Body was bent 3 ic follows, that this ought to make the
Body grow ftreight again. And chus we experience the
Property which is called Stiffnefs, and which Workmen
cali I the Power of Spr'mging.

38. However, this Property ought not to be found in ’i'^-Whykh
all Sorts of hard Bodies indiíFerently 3 becaufe there are "aiihárdE^-
fome, whofe Pores are fo large, that though they be día.
ftreightened by bending the Bodies, yet they will be ftill

I. The Power of Spritt^in^') Sínce
this íubtil Macterj as was betore pro-
vedjisnnly fiñitiouS) ic is much more
probable, that if a Body ba compound-
ed ot fuch Sorc of Partidas, thacic be
compaóli and bcndi or yields inxoard
fo Prejpim wiíhortt any fiding of his
Partsj it is hard and elafick^ reutrn-
ing to its Figure with a Forcé arF
/ro?M the mutual jítlraSiion of
its Parts» Newf» Opt. pag. 370.

But if che Parts of the Body ílíp
undcr one another, chen the Body is
of chut Sorc, which will yicid to the
Stroke of a Hammer s .But con-
ccrning che Laws of che Communi-
cacion üf Motion, in fuch Bodies as
have a Power of fpringing hack, or
are Elafid\ as they cali ic, when
they- mcet others, w'ichcercain Forces.
Sffi the Flotes on Chap. xi, 6.

wide

/

S 34

-9.

jBlate of Irort
becomes Ela-
Jticky by being
beateriy wheu
it is coldf

4.0. Ha-o thls
Poivír may be
hJÍ 9

4 1 . Wbence
thá Forcé wlth
Kohich a
Spring mf'
hsnds it fálfi
Arijeí*

4?. P^b'by
fams 'Bodies
"preak in rty
Jioring theri}'

JilVf!-

RO H A U LT’s System Pait 1,

wide enough to give a free Paílage to the fubcle IVktcer.
Thus we can perceive by our Senles, rhattheParrs of^teel
which is noc temperedj are larger, and conlequencly the
Pores widerj rhan thofe of tempered Steel ,• whence it is
eafy to apprehend that the Pores may be ftreightnedj with-
out hindring a free Paffage of the fnbtle Matter through
them ; yvhence it follows, that when it is bent, it will not
fpring back again,

35). Now to lliow, that the Fowerof Springing coníifls
intirelyin the Imallnefs of the Pores of a hard Bodyj let
US confiderj that if a Place of untempered Steel, bebeat-
en upon an Anvil when it is coid, it will acquire a Pow-
er of Springing which it had not befóte- But it is mani-
feñ, that this Beacing does nothing elfe but make the Parts
approach neareroneanother, and by this Means ílreightens
the Pores : whence it follows, that herein conüfts this
Power.

40. Itmay befurther obferv’d, that if a Spring be held.
bent a long time, without being allowed to recover iT
felf, the fubtil Matter will be forced to alter its Figure
by growing longer, if it be not abk to wear in pieces the
Matter of the hard Body ; or if it he, the Pore(^k:VÍll grow^
bigger and bigger, fo as that the Matter of the Firíl: and
Second Element may país freely through them ; and this
is the Reafon why the Body ought tc^.pfe the Power of
recovering it felf, in proportion as it is capable of being
worn ; which agrees v/ith Experience.

41. The Forcé wich which a Body unbends it felf, de-
pcnds pardy upon the Swiftnefs of the Motion of the fukfil
Matter, and partly upon the great Number of Pores through
•which it pajjes at a time : But it depends chiefly upon
the Difpoftion of thefe Pores as they become mfenfbly flreighter
and flreighter. For by this means, that which gets into
them ought to have the fame Forcé, and to produce the
farae EfFed:, as a Body which paíTes between two others
whofe SuperScies are almoft parallel. Now according to
the Laws of Mechanicks, though the Body which thus
palies between two others be very fmall, and moves but
ílowly, it will notwithftanding, have an incredible Forcé
to feparate thofe two from each other.

42. When che fubtil Matter begins to remove the
Parts of the Body which are in its way, it has their whole
Refiñance to overeóme, and alfo fome of the Refiftance
of the furrounding Bodies ; Now becaufe every Thing
endeavours of itfelf to continué in that State in which it
once is, and eberefore the Bodies which haye received a

cercain

43 *

and Chat on che con- “3,*:

Chap, 22 , of Natural Philosophy. 135

ceitain Motion^ continué of chemfelves in that Motion ;
this fubtil Matter cannot continué to impelí them;, but ic
muft increafe their Motion j and ic may fo happen, that
by ics impelling and moving them in this manner^ it may
fo far divide che Parcicles of the Body, through which ic
paíTeSj from each other, as entirely to feparate and break
them j efpecially if the Body be brictle.

43. Now in order to underftand how ic is, that fome
Bodies will beiid wichouC breaking,

traryj others will very eafily break: ic is to be obferved, 'tiéncfs'^f 'a
that the Texture of fome may be fuch, that their Par- "Soiiy mijifls
ticles may be intermixed wich each other:, like the Rings
of a Chain, or the Threaí^ of which a Cord is compo-
fed. Now ic is eafy to conceive, that thefe Bodies may
be wound feveral times round without' breaking, becaufe
their Partióles are fo hooked togetber, that they may be
bent any way. On the other hand, there may be Bodies
•^lich are not of fuch a complicaced Texture, which are
hard only, becaufe their Parcicles touch one anocher in a
few Places : V/hence it follows, that one cannot fe-
parate th^ ever fo litcle, but their whole Continuity
will be dsfcroyed ,• and thefe are what we cali brift/e
Bodies.

44. Leather may íérve for an Inftance of 3 Imber Bo-
dy, that is, of a Tjiy that will bend without breaking ;
and Glafs, on the other hand, for an Inftance of a brittk ¿,7
Body i that is, one that will break before it will bend ; iríoíí, ¡s
And there will be no doubc, but that the Lmhernefs of 'ZndthTttfa
the one, and the Brittlenefs of the other, coníifts in what hrinu Hody
I have faid 3 if we confider che Place where a Piece of /’»““'*•
dry Leather is pulled afúnder, and the Place where a Piece

of Glaísis broken: For the Leather appears unequal, and
as it were untwifted, which is an evident Sign, that the
Parcicles which are at the End of one Pare, entered in
between the Parcicles which are at che End of the other
Partj and onthecontrary, theBreachof che Glafs appears
very well poliftied, which isa Sign, that the Parcicles of one '
of its Pieces, touched the Parcicles of the other Piece on-
ly, without entering in between them.

4<j. If Glafs, which is very brictle, have very large
Pores on one Sideof its Superñcies, and which grow lefs Giajfes nm-
and lefs towards che other Side ; there cannot enter into
thefe large Pores, fubtil Matter enough to lili them, but ^J^tthLt t‘Jn^
that by continuing its Motion very quick towards the meítdhdwhh.
ñreigheer Parts of the Pores, it muft wholly diíunite the
Parts, . Now ^hen a Drinking-glafs, which is juft made, .

K 4 grows

44. Why the
Place in
vphich a Um^

i

*13^ RO H AU L'^’s System Partí,

grows coid on a fuddenj it is impoífible but thac the-
Pores muft be larger where the Glafs is thickeíl ; becaufe
the Bleatj which dilate Bodies, continúes longer here than
in the other Parts ; Wherefore the fubtil Matter which
enters into thefe large Pores, going on fwiftly, and with
great Forcé, i muft break the Glaís in the Places where
the Pores are feníibly leís. And this fo commorJy hap-
pens, that it is fomething ílrange, if a hundred GlalFes
be expofed to the Air as ibón as they are made, if one
of them efcape without breaking.

'46.Tohitider The Glafs-makers have a Way to prevent this In-
Z^fireal™ convenience, by putting the new-triade Glaííes into the
Arch of the Furnace, where they are removed by little
and little out of the Fíame, íb as not to get above the
Space of nine or ten Foot, in íix Hours time, and then
they are expofed to the open Air, and fo all the Parts
growing infenfibly Coid, the one as well as the other,
the Pores become equally ílreight every where, and the fub-
til Matter which can enter into one of them, can ruf'
I from thence freely, through all other Parts of the Glafs

where the PaíTages are equally open.

^n^Pr"^¡n concerning thg^^Caufe of

Tfieíafs^ Glaííes- being broken as it were of themíélves,' opens a
Vrop. Way for us to explain a kind of a Miracle in N ature,
which was iately difcovered and bmught hither from
TaE,iii. Ho¡¡a?íd¡ and which has travelled tlmough all the Uni-
verfities of Europe, where it has raifed the Curiolity, and
confounded the Reafon of the greateft Part of the Phi-
¡ofophers. It is a kind of a Drop of thick Glaís, and fuch
as the Glafs-Windo'A's are made of, near the fame Shape
and Bignefs as defcribed in the Figure. It is entirely So-
lid, except perhaps we may fomecimes fee a few fmall
Bubbles of Air in the thickefl: Part of it, as at D, where
it will bear pretty hard Blows of a Hammer without
breaking. And yet, if the little End of it be broken off
any where near B, the whole Body will buril in Pieces
with a bíoife j and we íliall fee it fcatter it felf all round.

I. Mí¡Ji break the- Gíafs) But íc
may be (and it is more iikely) that
che Coid, by ítopping the Motion
of í(jme of the Parts orí a fudden,
whilít che reft are in great Mocíom
breaks Vefíéis made of Glafs. For
thus airnoft all Bodies are broken by
che unequal Motion of their Parts :
Henee a Tile by one Blow burfis a».
üini?r many cimes into ewd or efarge

hundred Pieces. Henee the Chymiíls
Veílelsareofcen broke. Henee they
whü cuc Drinking-Glafles into Spi-
rals, firíl puta red hotiron near them,
and chen pour coid Water on tha
Pare of ihe Glaís which is heared.
And henee Drinking-Glaíles are re-
porced to be broken only by the
Voice beiiíling them»

and

Chap. 22. o/ Natural Philosophy. 137

and to a good diftance, in. a Powder, which thcugh ve-
ry fmalL has its Parts cracked in fo many Places, that
it is eafy to divide them by preíFing them between ones
Fingers j which may be done without any Danger of
pricking them, as there is, if we íhould handle a piece
of Glafs fo, after it is powdered in a Mortar.

¿j. 8 .' To fay the Truth ,■ this 'Pha?io??ie}jon is fo fingular, 48- 0/ the
that ieis no wonder it íliould at firfl Sight furpráe us. ‘cXfeofthe
But if we confider it more clofely, it is eafy to obferve, Motionofthe
that there is nothing elfe appears, but only the local Mo-
tion of the Parts of the Body, which are carried from
the' Center to the Circumference : Now as we cannoc
conceive how a Body ihould begin to move of it felf,
without being put in Motion by another Body which
was in Motion before j fo it is eafy to imagine, that the
fcattering about of the Partióles of the Glafs-drop, is
owing to fome Matter which gettinginto its Pores, preíTes
imon them and divides them, in the fame manner as we
® a W edge when it is driven into a Body wich great
Forcé and Velocity, fplits it, and feparates the Parts from
each other. And there is no Doubt at all, but that this
is the fanTji Matter which breaks the Glaffes in the Glaís-
Houfe, when they are fuffered to cool too foon.

, 49. Now in order to underftand how this Drop could 49, Whai

acquire a Difpolition proper to produce this EíFeá, there
is Reafon to guelsfThat the Workman, who makes a Se- thfpán”of
crétof it, has a Way of cooling it all at once, by dipping the Drop
it whenit isvery hot into fomeSort ofLiquor, which hin-
ders it from breaking in pieces ; For we fee by Experience,
that Glafs which isfo co'oled in Water, breaks into fmall
Pieces. But be this Liquor what it will, it is certain, that
the Parts of the Droj>, which are neareft the Surface, cool
firft, and by communicating their Motion to this Liquor,
lote what they had before, which kept them at a little di-
ñance from each other j and fo they are condenfed, and
contradi their Pores, and fit them to the fineíl: Parts of
the fubtil Matter, which preferves its PaíTage through
them. But this is not the Cafe of the internal Parts of
, the Drep, which not being cooled till after the other,
cannot contract themfelves fo, becaufe thofe other being
grown hard, pnd difpofed like an Arch, do not at all
prefs upon theín ; fo that the Pores which are amongft
the Parts neareft the Middle, are large, and grow lels and
lefs as they come towards the Superficies. And this be-
ing allowed, there is a plain Reafon for what caufes fo
great Admiration. é

50. It

ijS RO H AU Ll’^s System Partí.

cJhtíobllt wonder that che Droj> will bear che Blows

ibí 3W o/ of a Hamrner, becaufe it is chick enough for that ;
«Hammer. For ocher Pieces of Glafs of che fame Bignefs will do
che like.

si-Thatthty It is alfo manífeftj that they ought not to Break
themfelves^ as che forementioned Glaíles do, be-
íbemfeives. caufc che fubcil Macccr which paffes chrough chem, finds
as free a Paffage to come out, as to enter in.
f¡c^i^]Zcs Pnd is broken ofF near che place

■ marked B, we can there fee very large Pores inco which

che larger Partidles of che fubcil Macter encring in a great
Quantityj and concinuing tomcvefromthencevery fwift-
lyj, towards every pare of che Superficies, where che Pores
grow ftreighter ; chey cannoc buc i leparate every way che
Parts of che Glaísj and fo divide them inco that Powder
which we fee.

J5. ri'hyit y 3. This Truth is confirmed by obferving, Firñ, That
tip‘¡Z‘s‘’ 2 t Extremity of all, which is at A, is lo fmall, th|
the -very End thcrc could be 00 fcnfible difference in cooling becweeá
of eiiis hn~ the infide and che outfide, fo tliac che Pores there are of
"xab. III. sq'Jal Bignefs throughouc. Wherefore if che End be
lig. í* broken off thereaboucs, this will not give leií'/e to che
fubcil Matcer to lee in its groífer Particles, any more than
if it were noc broken at all, and confequently che Dw/
oughc noc co buril in Pieces 3 as by^ Aperience we find
ic does noc. '

54. Thatthe . y 3.. Furcher, if one of thefe Glals Drops be made red
Zgahi, ™ fufíered to cool ílowly, its

os¿ht to hfe ’ Pores will then become very néar equal, in like manner,
i^prtas of ag Workmen neal Steel. After which, if che End ofthe
fmdTrf be broken oíF any where, becaufe there can no fub-

til Macter enter in, buc fuch as can go ouc 011 all Sides
wich as great Eafe as ic entred in, therefore the Drop a ought
noc to buril in Pieces at allj which alfo we find crue by
Experience.

I. Sepárate every way) Becaufe
Glafs is a Body which has a Power
oí Springing,- iris probable? that this
Glals Drop is broke in the fame
manner, as a Steel Bow burfls in
pieces íbmecimes, when it is loofned
on a fudden i v/z* by the too great
Celeriiy and Forcé of that Moción
which arifesfrorg the mutual jrictra-
ilion of its Parts. For its Parts
from the Center to the Circiimi-e-.
rence» feem to be iikefo many Bows

bsne And henee perhaps ic is, that
afeer ir is buril in Pieces, its Fiflures
are difpofed like íb many Radii
drawn from che Axis co the Superfi-
cies, as Mr. Hook obferved in a
Glafs Drop covered over with Glue.
SeeHook’s Mkrography Obferv, jth,
z. Oítght not iobnrji in pieces) For
che fame Reafon, that there is no
danger of breaklnga Bow when ic is
gradually loofned.

Chap. 2.2. o/ Natural Philosophy. 139

<5. Laítly» To confirm .what has been faid of thc In- í/. Sme m.
equality of the Pores which are in the Middle^, and thofe "“(/o/Lá-
near the Superficies in thefe Sort of Drop, I carried three fidane,.
of thern to three different Lapidarios ; Tlie firfl: of them T^b. iii.

I ordered to cuc the Drop which I gave hitrij with Pow-
der of Diamond about the Place C. I ordered the Se-
cond to drill a Hole in his, with the fame Powder about
D, and I ordered the Third to put his upon the Wheel,
and grind it plain at E, with Powder of Emery : Now
after thefe three Workmen began feparately to work up-
on them with as much Caution as they do upon Pearls or
Stones of a great Valué, and had grouiid with thefe Pow-
ders as much ofF from the Drops as amounted to the Thick-
nefs of a ¥rmch Two-pence, which I reckon is as far as the
fmall Pores reach, I faw each of them buril in pieces as
ufual, to the great furprize of the Workmen, who did' not
at all espeót any fuch Thing.

^56. But to return now to the Confideration of Liquids. 56. Oftao
obferve firíl, That if they be all reduced to two Species,
the one complehending all thofe which we cali ihm and ulmr¡T
the other, all thofe which we cali fat, it will not be dif-
iicult to áltermine what their principal DifiFerence con-
fifts in. For lince the former is very eafy to evapórate,
but the Latter evaporates with great Difficulty, we can-
not but think, thávjjie Partióles of the one, muft be of
very limpie Figures to be able to diíingage themfelves
from each other, and the Particles of the other, of more
entangling Figures, fomething like B ranches of Trees, by
which they hold each other together.

57. And this is confirmed from henee, that if a Vef- 5-7. w'íy
fel full of ihifi Liqzíor be fo inclined, as to pour it out Water, nhm
llowly, the Liquor will run about, and divide itfelf into a f/s/perfuUt
great -many dillindl Drops ; whereas if it be a fat Li- Drops,
qmr, it will go on in a long Thread, whofe Parts are un-
interrupted.

58. This being fuppofed, we fhall not think it at all ¡i. why
ftrange ; that Oü or Air is Ib hard to mix with Water ;

The Realbn of which is, becaufe the Particles of thelé
Liquors unite together much eafier than they do. with the
Particles of the other ; Whence it is, that if Water and
Oil put into the fame VeíTel, be fo fhaked up together,
that they feem to compofe but one Liquor, they cannot'
continué fo long, before the Particles of the Oil which
meet each other, will entangle themfelves fo as to com-
pofe feveral Drops, which becaufe of their Lightneís, rife
yp, at the fame tiene that the Particles of the Water, whofe

Motion

•aillnot mix
together.

14 ^

59 * TJoat the
Drops of 07te
LiqHor xchich
fvoirn irt ano-
thtr Liquor^
are round*

6o. The Opi-
tiion of the
Ariftocelians
coricernirj^
the YOtmdnefs
ofíhefeDrops»

6r. A Con-
fíitation of
the Opinión of
the Ariüoce-
liaii5.

-ROH AULT’ s System Partí,

Motíon caufes them alio to meet, join togecher likewife,
and compofe other Drops which fink downwards : And
rhis is the Reafon why thefe two Liquors entirely clear
themfelves of each other, and become diftindtj the one
at the Top, and the other at the Bottom.

59- It is worth obferving^ Úisiihe Drops of Difíors,
•which fwim m a largo ^antity of other Liquors •whkh
theq •wÜl not inix •with-, are all round, like Balls. This
cannot be perceived i'n Drops of Rain as they fall in the
Air^ by reaíbn of cheSwiftnels of their Fallj on the con-
trary, they ought rather to appeair long, as we Ibould
take them for ünall Coluinns ,■ for the fame Reafon that
a lighted Torch moved quick, appears like a long Train
of Fire. A better Way then for us to take^ in order to
fee if the Drops of Water which fwim in the Air be
round:, is to put a little Water into the Hollow of one’s
Hand, and to throw it up into the Air^ about the Height
of our Eyes ; for then it will divide it íélf into a greac
many fmall Drops, which beginning to defcend ve^
flowly, give the Spedator an Opportunity pf obferving theit
Figure.

6 o. This Thísnomenon has always been obfejj^^ed, and a
Reafon for it attempted to be given, by faying, that the
Parts of the íárae Liquor have a mutual Affé&mt for each
other j whence follows a Defre ofuniting together., which
cannot be done perfeólly, but by compofing a Ball ,• for
if they compofed any other Figure, thoíe Parts which
were moft diftant from the Center, would tend towards
it with a greater Forcé than thofe v/hich are nearer it, and
confequently.make them give way, and remove back till
they are all equally placed about the Center, and fo be-
come round.

di. But becaufe theíe Words, AffeBion and Defre
no Meaning, that we can apprehend, unlefs they be afcri-
bed to Subjefe which are capable of Knowledge j there-
fore we cannot apply them to the Parts of Water, with-
out fpeaking very improperly and obfcurely. Wherefore,
thefe are fo far from explaining a Thing which ought to
be very eafy, (for we are only inquiring into the Figure of
a Body j) that they perplex it with Terms which have no
clear and diftind Signification when applied to fuch Sub-
jefe. Further, let this Defre of uniting be explained how
it will, it is very abfurd to afcribe it to Subjefe which
feeni naturally to be ntted to difunite from each other.) be-
caufe Nature has made them lo capable oí difuniting.-

da. In

y

Chap. 22. of Natural Philosophy.

62. In order then to find out the Caule why the Drops of
Liquors which fwim in other Liquors are round, we muñ
.keep this Truth in our Minds : That everj Thing endea-
vours, as mudo as it can, to continué m that State, in
'oihkh it once is; and confequently, that which is once in
Motion, would continué tomove with the ítíOísDetermina-
tion with which it began, that is, according to what was be-
fóte faidjin the faene ítreightLine. ThuSjif theBody A, for
Example, is moved along the Line AB, it is determined
at the Beginning of this Motion to go towards C, and it
will never of its felf tend to go towards E or towards D.
However, if when the Body. is come to B, it meets with
any Obflacle there, it may turn out of the Line BC and
go in fome other Line. But becaufe it is forced out, it
foliows, that it will go as little outas it can, thatis, when
itquits the Line AB at the Point B, it will tend to move
in a Line which will make the leaft 4 ngle that can be
cfliiceived with the Line BC. And becauíé the Line BD
does not make fo fmall an Angle with the Line BC as
BE doesj we cannot but think, that the Body A tends ra-
ther to move in the Line BE than in the Line BD. And
becaufe tlil Circumference of a Circle, of which BC is
the Tangent, makes a lels Angle with BC than any An-
gle comprehended betwixt two ílreight Lines ,• we muft
conclude, that the,.:,>Jody A, when it is arrived at the
Point B; will reíift'turning into the Circumference of a
Circle lefs than into any ílreight Line. Laílly, Becauíé
it is certain, that the Circumference of a great Circle makes
a lefs Angle with its Tangent, than the Circumference of

|, a ímall Circle does with its Tangent, we muíl alfo Con-
clude, that the Body A, when it is arrived at the Point
B, where it is forced to turn out of its Way, will refiíl
ftilllefs, the deferibing the larger Circumference BG, than
the fmaller une BF.

63. This being fo , if the Particles which compofe a
Drop of Liquor, and which are hindred from going on
in their Motion, by the Liquor which furrounds them, be
compared to the Body A j and all that has been faid of
the Body which made Refiílanceto itat B, be applied to
the Particles of the furrounding Liquor, which do not
make fo ■ great Reíiftance, but that they can retire back
a little ; we conclude, that the Particles of the Drop, do
gradually remove thofe furrounding Particles which get
lyithin the Sphterical Superficies which the Drop may be

com-

141

62 ,T'/jaí So-
aVef Tobich are
compdled out
of the roaji
tendrather to
' deferibe the
Circuwference
of a Circle
than a
Jirei^ht Linea
and the Cir-
cnrnference of
alarger Ch-
ele rather
than of a
fmaller,

Tab. III.

Fig. 6.

C 3 . IVhy the
Drops of Li-
quors are
round.

142

^ 4 »

DropSt any
tony f 7 ipport-
edi o«ght to
be a liítlé

RO H AULT’s SYS.TE.U Partí,

comprehended under : And becaufe i the World is full,
and the Parcicles which are removed out of their Place,
have no where to go, without removing as raany others,
they muft neceíTarily be driven to thofe angular Parts of
the Drop which are without that Iphxrical Superficies;
and lo the Drop will of it felf become of a round Figure,
though the furiounding Liquor contributed nothing elfe
to it, but only not reiifting it at all ; But becaufe the Par-
ticles of this Liquor, are more hindred from continuing
their Motion in a ftreight Line, by the angular Parts of
the Drop, than by the others which are nearer the Center,
it is evident, that they muít forcé them towards the Cen-
ter, and at the fame time make thefe other remove fur-
ther offfrom it. ^ And in this manner the furrounding
Liquor contributes as an efficient Caufe, towards making
the Drop round. Nay, we may affirm, that it does the
greateft Part towards it, if, all other Things being alike,
this be moved with the greateft Celerity.

64. But it is to be obferved, that there are tw'o Things
required in order to make Experience agree with this De-
monftration : The Firft is, That the furrounding Liquor
be not more than ufually agitáted by any external© orce ; and
Secondly, That the Drops be not any way fupported, at
leaft, when they are of any confiderable Bignefs ; for then
their Weight, which is fuperior to tl^Caufe which maltes
them round, will make them a little fíat, fo that they
will be round only in that Part which is parallel to the
Horizon. As we fee by Experience in Drops of Water

T. The World ' is futí) See the
Notes on Chap viit.

2.. xy4nd in this manner^ A Portíon
o£ any Liquor, incloíed in another
Liquor, which it does not mix with,
will preferve its Figure, whatever ic
be, without any Alceraclon, if che
Parts of the furrounding Liquor be
at reñ, with refpeft to each other. 1
See Newt» Princip. "Book II. Prop. 20.
Cor, ^th, But if the Parts oí the
lurrounding Liquor beagitated, che
inclofed Drop muft neceíTarily be ,
comprefled into a .globular Figure. ¡
For fince che Superficies of any other
Figure is greacer than that of a ;
Globe, and cberefore expofed to '
more Aitacks from the Parts with
which le doe's not mix? comíng up-
en it on all Sidesj and becaufe
whatever is prefled upon on all ,
bidés, retires tbither where ic
may be leaft prefled upon j it is e- ¡

r-

videnc, that the Parts of the inclofed
Drop, muft gacher chemfelves into
the Form of. a Globe, when they
will be leaft prefled upon. And chis
they will do, jf there w^ere no Iiich
Thingas AttraSfion. Bucíincethe
Drops oí Water and of other Li-
quors, gather chemfelves into a round
Figure, in a Faenrím, as well as when
inclofed in any Liquor? che Caufe
of this üught by all means to be af-
cribed to the mutual Attraflion which
there is betwixt che Parts cf one
and the íaine Liquor. (See the NotfS
on Chap, vX.u^rt. if.) Por the Drops
of every Finid a^e¿l a rotmd Figure^
by the mutual KAttraÜion of their
Parts: In the fame manner as the
Globe of the Earth and the Sea af
fcéls a round Figure^ by the mutual
^ttraílion of its Pa.rts by Gravitj'
Newc. Opticks, pag. 37c.

which

Chap. 22. o/NaturalPhilosophy: 143

which reft upon fuch Leaves of,Herbs as they will not
wec, and in thofeput upon a dufty Tabla,- as alfo in Drops
ot Oil or meked Greafe fwimming on Water, which in-
deed are not round, but only on that Side which is level
with the Horffion, for on the other Sides, they are flatter
in proportion to their Bignels and Weight.

6^. This laft Oblérvation ought to be underflood only ¿f-
upon Suppofition, that all Things elfe are alike. For it
isnot at allitnpoffible, but that of two Drops of difFerent are more
Liquors, that which is the moíl: heavy, may be the round- romdthan
eft, provided it be alfo the fmallefl: ; The Reafon of which
is, that all the Particles of the Liquor which furrounds
the Drop, do not help to make it round, but thofe only
which are applied to the Surface of it the reft, which
enter into the Pores, help rather to diíEpate it. Where-
fore a Drop, which is fmaller and heavier, having its Pores
lefs, and perhaps a leís Quantity of them than the other,
which is iarger and lighter, has alfo its Surface more con-
tSued, and confequently gives more Opportunity to the
Caiife, which makes it round, to work upon it, and lefi
to that which would diíEpate it. Thus we fee, that a
Drop of (%ickfilver is always more round than a Drop of
Water a littie lighter.

66. On thecontrary ,• Spirits of Wine, being very light,
muñ have ío manyjTores, and the Superficies of it muft
be fo interrupted,'''*hat there can be but a very few
Particles of the Air applied to it to make it round, the
greateíl part of them país through it, and tend to diíE-"^
pate it,- alio this is a Liquor, which it is very diíEcul’t to di-
ñinguifli into Drops, as may be triéd, by putting a littie
of it into our Hand and throwing it up into the Air ;
for if it be well reéiifed, it will not fall down in Drops,
as Water does, but it will be lo diíEpated by the Air,
that none of it will appear íénfibly on the Ground. So
alfo if it be thrown upon a dufty Table, it will not gather
into rounds Drops, but ípread it felf about, and mix
with the other Bodies which it meets with ; nay, even
with Soot it felf, which Water will not moiften.

6y. Having thus íhown what kind of Superficies that 67. vny a
which is common to two Liquors, the one incloléd in the
other, is j it may not be amifs to ftop a littie, and examine S'w
what fortof Superficies that ought to be, which is between others.
two Liquors, the one con tained in a Veífehandtheothernot:

But becauíe there may be lome DifFerence in this, according
as the VeíTel will be wetted or not wetted by the Liquor
contained in it ,- it is to be obferyed, that a Liquor there-

fore

■ "^44 RO H AULT^ s System Partí,

fore wets a hard Bodvf bccaufe it immediately touches its
Superficies; and thatanocher Liquor does noc wetit, bf-
caufe it does not immediately rouclí its Superficies ; but
there is room left for the fubtil Matter to país between thc
concave Superficies of the one^ and the convex Super-
ficies of the other.

68. Thatthe 68. This being fuppofed ; vve conclude firíl, that if a
Glafs; whofe upper Edge is oí an equal Height
JdemiGiafs aH round, be exa¿ilj filled with VVater, the Surface of the
exaaiyfiiUjs Watcf will be perfedlly level and plain ; becaufe the Air
^mtefiat. touches icj does liot preís more upon one Part than

upon another.

6^" Glafs be not full of Watcn the Superfi-

cics ought to be Concave; * becaufe the Air which comes
mhich mili in at the Mouth of the Glaís, and circulares about the
^^afs and the Water, as if they were one continued
Thing, cannot fo ealily tura to move along the internal
Superficies of the Glaís, as continué its Motion in Ae
Middle: From whence, being to go out again at rae
Mouth of the Glafs, it defcribes a Curve in a contrary
Pofitiom'to what it did when it entered in, much the
farae as is defcribed in the Figure ; fo thatthe Water
is preíTed more in the Middle than on the Sides, and conle-
quently muñ rile tovvards the Sides.

yo.. Experience would perfeiítly a^f;e with t’nis Reafon-
pcrficksisnot cohvenient Motion for

onght to be
Conc'jve, if
the Glíjfs is
not ftilL

Tab.III.
Fig. 7.

70. Why the
Concnvc Sn^

J^hericaU

yj . That the
hoUow Snr-
facc of the
ÍVater in a
fmall Tiihe
r.otfnlU is
Jpherica/,

Tab. III.
Kg- 8.

the Air is in a Circle, it fhould feem, that it ought to
bend the Surface of the Water into the Form of a Con-
cave vSphere, which yet it does not do ; For the Sur-
face of the Water is curved only towards the Sides, and
is perfedly level in the Middle. But the Reafon is plain;
for if the Glafs be large, a great Quantity of Water muft
be raifed up to make the Curvature fo convenient, as
the Water requires, which it is certain is refifted by its
Weight.

71. And for Proof of this; If into a fmall Tube of
Glaís, in which a fmall Quantity of Water rifing at the
Sides maltes irs Surface fpherical, íbme Water be poured,
fo as not to fill it ; w'e may obferve, that it will con-
tinué in the fawe matmer Sphe-i‘ical.¡ though the Tube be
inclined as you íce in the Figure, where the Curvature

I, "Becaufe the Air) Since allthefe
PhiEnomena are the famein a Vactt-
tiUi as in the open Air j we muíl
aflert, that che Superficies of any
X-ifjuor conuined in any Veíl'el is

Gibbous or Concave^accordingas the
Particles of the Liquor are more or
lefs mucually actra£ted by eachoiher,
than they areby the Maccerof which
che Veíle] is made.

ABC

Chap. 22. of Natural Philosophy. Í45

ABC repr^íéncs thé Surfaceof the Water, which is there-
fore aboye the Leve!, and manifeftly higher at A than ac
C, becaufe thac Poíition of the Water agrees better with
the Motion of the Air^ which would be more turned
back. and with greater Forcé in the Place D, if the Wa-
ter were more upon the Level DBE.

j2. The fame Caufe, which hinders the Water from yi.vFfya
growing level in an inclined Tube, hinders a Bottle alfo "^ottiewith a
I which has a very flreight Neck from emptying it
\ when it is near inverted, and the unequal Height of ihe fanófmter,
two Parts of the Water which endea vour to come out at
I the fame time, Ihould feem to deñroy the ¿squilibrium oí Zmjupaardt,

' the Air’s PreíPure, which repels and fupports it by its mUnofempty
; Weight. ForExample ; Though in the Bottle here de- m..

; Icribed, the Height of the Water which endeavours to Fíg. 5.

I come out of the Bottle at C, is greater than of that at A,

I and therefore Ihould feem to be able to forcé the Air to
defcend at C, and to rife again by A, and get into its
iSace j yet this docs not happen, becaufe the Parts of •
the Air now defcribe the Curve ABCj and the Dif-
ference of the Weight of the Water at A, above that
at C, is very fmall, thac it is not able to make the
Air to delcribe a Line thac is more curved, as it muft
doj if the Water which defcends by C, took uppartof
the Width of the Neck.

75. Ifalittlemdíí Water be pouredinto aGlaís ofthe 73- That the
common Shape, than will fill it exadtly fullj as thac which
would run over the Sides, is more expofed to the Power whe,, the
of the Air than any other Part is j it follows, thac the
Air ought to pulla it back towards the Middle, where it tmvJ.
ought to be higher, in order to its more convenient Mo-
tion. And thus we fee that a Glafs may be filkd heaptng
full, and thac the lefs the Glafs is, the nearer does the Su-
perficies of the Liquors it contains apprOach to a Sphere ,•
becaufe itdoes not fuílain the Weight of ib great a Quan-
tity of Water, and the Forcé of the Air is fuíHcient to
bend it. in this.

74. If the Glafs be greaj}, or for any other Reafon will ' 74 - That the
nothemadeivet^ whatever Quantity of Water be put in-
to ir, » the Superficies ought always to be convex,- be- GujimifuU,
caufe its Figure does not ib much depend upon the ex-
ternal Air, asupon the Air thac flows becween the in- '^nulevght

/7//0 to be £o«-.

I. Tbe Síipcr peles ought alxoíiys)
Thus the Superficies of í^ickíilverin
Glaís Tubes, is always ^bous, be-
caufe it doej non wet che Glaís but

L

in VeíTeís of Gold that arenotfulí,
its Superficies is concave, as chacot
Water is in Glaís. imites ou

Art, 69. abovi*

nal

34^ RO H AULl’^s System Partí,

nal Parts of the Glafs, and the external Parts of the Li-
quor which it contains, which by its continual moving
round, blunts the external angular Parts which reíiít its
Motionj and forces them towards the Middle, or elfe
forces them inwards, and fo caufes the Water to raife ic
felf up towards the Middle, wherethe Air oppoles its Pat
fage lefs, becaufe it cannot get thither, but by altering and
bending its Courfe.

foregoing Ar-

ia¿mthe°np ticleSj wc infer^ that the Air which depreíTes the Middle
{ftheU'^^iir, of the Superficies of the Water in a Glafs not full, ought
frcJtheMd- Caufe, to drive light Bodies which fwim

¿/e lo the upon it, and touch it immediately, towards the Sides :
Sidcs. Xhis I have experienced in fmall Globules of Glaís full of
Air and clofed up» which an Enameller made as light for
me as he could ^ for thefe being put towards the Middle
of the concave Superficies of the Water in a ílrait
Glafs not very fulL it was very pleafant to fee them dü-
*ven from thence to that Side of the Glafs which was ne*-
cfl: to them.

76 That this Becauíe I made ufe of a fmall Globule of Glafs, and
^aufidhyAi- ^ Veílel of the fame, in this Experiment, fo(íe Perfons
traíiim. perhapsj may imagine, that this Globule moved towards
the Side, becaufe it was attrafted by the Glafs : But it is
very eafy to confute this Imaginatio|^ for not to men-
tion the Obfcurity of that W ord, me fame Thing will
happen in a Veffel of Wood, orofany otherMatter what-
loever, ' which we cannot fuppofe to have any Sympatby
with the Globule.

which evidently overthrows this Opinión,
cálht to go and confirms that which I have advanced, is, that if At-
framtheSides craótioH had any Thing to do here, the Globule ought to
raove fwiftly from the Middle to the Side of a convex
chfiheafwg Superficies of the Water in a Glafs heaping fullj for
ptii. beíides the Attra£tion, the Declivity ought tohelpitsMo-
tion. Which yetisnot fo; but on the contrary, it moves
from the Side towards the Middle, as it ought tb do, if
what I have aíErmed be true ; becaufe, as was íáid be-
fóte,' it is the Sides which are moft expofed to the Forcé
of the Air ; and the fame Caufe which drives the Water
from the Sides to the Middle, ought alfo to drive che fmall
Globule.

I, Whiih we canmt fnppofe] See the Notes on Chap, xi. Art. ip-

Chap. 22. ^ N ATURAL PhILOSOPH Y. Í 47

78. But it is to be obíerved in thefe Experiments, that 78- Whya
’theBody which floats on the Top of the Water, muft healur^-hm

icnmediately touch ¡c, or which is the fatne Thing, muft Waur, -ahen
be wetted by it, that the Air may be forced to
round them both, as if they were one continued Body. jvaieVLes
But if the Body which floats on the Water does not im- thscmtrary
mcdiately touch ic, or is not wetted by it, we experience
the contraryj that is, the Body will defcendj 9 ow the Sides Giafs.
tovjards the Middle-, when the Superficies of the Water is
micave, and from the Middle tatuar As the Sides when the
Superficies is convex, becaufe the Parts of the Air which
pafs under the Body deprefs the Liquor all round, which
produces the fame Effeót, as if, when a large heavy
ípherical Body was fixcd upon the Declivity of a Moun-
tain, we íhould take away the Earth equally all round
it, and put Leavers under it to fupport ic ; for it is evi-
dent, ic would by that means be difpofed to defcend tothe
®ttom of the Mountain.

79. It is to be obferved further, that when a Body
which weighs more than an equal Bulk of Water fwims

upón the ¿water, as a Needie madeor Steel will do, the pat «ponthe

Reafon oHt is this,- that the Air which preferves it felf

a PaíTage between the Body and the \Vater, fupports it

and hinders it from ftnking : For we ought not to think

that ic proceeds froúi henee, that the Parts of the Water

are harder to be feparated near the Superficies, than deeper

in, as we may be apt to imagine ; for having caufed fome

fmall Needles to be made of Glaís, which were lighier

than che Steel Needles of equal Bigneís, and laid'them

gently upon che Water, they always funk down to the

Bottom.

80. From henee, viz. that che Body dipped intheWa- So.WiyUs
ter will be inoiftned, or not tmifitied, it follows, that the

Water will rife up on the Sides of fome Bodies higher ^mtleSUef
than ic is any where elle, or that ic will be deprejjed low- »//<>«« So-
er^ The Reafon of the Firft is, becaufe the Air which
inoves from one Side of the Veffel to the other, and paf- thlm a Hule
fes over the Body, permits the Liquor to rife in that
Hollow which the Air cannot without great Difficulty turn
into ; Whereas when it paíTes under, as in the Second
Cafe, it depreííes the Liquor all round. And of this a
Miikitude of Experimenta may.be made, and an infinite
Number of them are made without any Notice being ta-
ken of them ; for cvery time we dip our Pen into the
Ink, we may obferve, that if ic be moiftned, the Ink will

148

Si» Vrhy the
Vf^ater rvlll
rifecotifidc-
rab!y in the
J^art whers
íRjo Pieces of
Glafs are fit-
ted to each o-
theV) roheyt
they are dtp-
pcd a [hile
into it»

8 ’ . Vl^hy the
P'f-^aterisfeen
to rife of i t
felf ¡n fnall
Glafs Tabes,

85. my it
does iiot rife
o» wíthout
End,

^í.Thata
^reater^an-
tityof y't' aíer
OHght to rije
in an incUned
Tnhe.

8 5:. Vyhy the
P’i^ater rifes
fomeitmes
hif¿her in the
fmnlícr-, than
in the Inrger
xAxm of one
inverted Sy-
phon,

Tab. I.

«S- 4 -

ROHAULru System Partí

rife ; and on the contraryj that the Ink is depreíTed about
the Pen if ít ¡s not moift.

81. If two plain Bodies which the Water will wetj fuch
as two Pieces of deán Glaís, be put very near one ano-
ther, and dipped a lictle way into aVeífel of Water; ' the
Air which inoves from one Side of the Veffel to the 0-
ther, in order to get over the Obílade that lies in its
way, ought rather to país over the Top of the two Glaf-
fes, than to defeend into that ftreight Place which is be-
tween them : So that the Water is not fo much preíTed
here as, it is in other Places, where the Air can go with-
out bending its Courfe fo much, and fo it ought to rife
to a coníiderable HeighC above the Level of the Water
contained in the Veífel; and thus wefee by Experience
that it does.

82. And there is nodoubt but that the Water would
rife ílill higher, if the two Pieces of Glafs were clofed
on both Sidesj for by that means almoft all the Air whiii
inoves crofs, without bending its Courfe, would be hi®
dred from entring in. Or, which is the fame Thing, we
miy take a very ítnall Glafs Tube open at both ,J!nds, and
dip it in the. Water, for then the Air cannot feSterin by
the Sides ; fo that the Water mufl: rife very high in fuch
fortofTubes, if they be very ílender; And indeed Ihave
made the Water rife a Foot highin a^lafs Tube fo fmall,
th«t one could fcarce get a Plorfe-hair into it.

83. However, we muft not conciude from henee,
that it ought to rife on without End in thefe fmall Tubes ;
for it.is eafy to fee, that the Water muft ftop, when the
Weight of that which is rifen, tends downwards with
greater Forcé than the PreíTure of the external Air has to
thruft it up.

8.}.. If the Tube be inclined, a greater Quantity of Wa-
ter will get in, becaufe, being fome way fupported by
the Glafs, it does not tend downwards with fo great
Forcé. Which is confirmed by Experience, accordingto
the moft exaft Laws of Mechanicks.

85. Having now explained the Forcé of the Air as 3
Liquid to impel Bodies which are clofe to it, we may
íáy with more AíTurance and Certainty than we could
before, what the Situation of a Liquor in an inverted
Syphon, whoíe Branches are of an unequal Thicknels,
as is here reprefented, will be. For Example, if we con-
fider only its Weight, we may confidently aíSrm, that

The Air which naves) See the Notes on Art. Sj, of this Chap,

if

Chap. 22. <?/ N AT'uRAL Philosophy. Í49

if the Water iii the larger Brancli, reaches up to the
Height AB, it oughc tp rife to the Height C in the little
Tube;, to beuponthe Level with the other ; But we may
add, that if this Branch be fo fmall, » that the Parts of
the Air cannot turn in it but with DiíEculty, the Water
will rife confidcrably higher than in the larger Branch,
fo as to rgach to D, according to what was now
proved. ^

8(5. There are few of thofe who enquire after a fer-
fetual Motmt, but when they fee this Experiment, for
want of rightly underñanding the Caufe of it, think
they have found out fuch a Motion. And indeed it
looks at firfb Sight very probable, that if we take one
of thefe Syphons, in the fmaller Branch of which the
Water rifes very high, and bend this Branch a little lower
than the Height which the Water rifes to, it might be
fo ordered, that the Liquor with which it is filled might
jun out into the larger Branch, in order to rife up a-
* ain in the fmaller one, and fo produce a perpetual Mo-
tion : But it is certain, that ^ they are deceived who
make this Conjefture ; for beíides that, the Branch of the
Syphon#out of which the Water is to run, ought to be
longer than the other, (which is not fo here, where the
¿lent Branch is in the Room of a whole Syphon ) it is
eafy to fee, that.f^ie Water, the Moment it endeavoürs
to come out at the End of this Imall crooked Branch,

is more expofed to the Forcé of the Air, than that which
is contained in the larger Branch; whence it follows, that
its PaíTage out muft be ftopped.

87 . This wñl appear moreevident, ifwe confider, that ^j^Thatina
when the End of the fmall Tuba of a bent Syphon, xlbeofwUA
whüfe Height does not exceed that, to which che Wa- hveryjmaii,
ter will commonly rife, be dipped into the Water, it will tktvyater
immediately be filled ; but if the End of the longer Branch ’Z^yTím'out

1. Tht 'Parts of the It

looks very probable, ac firíl ¿ightj
as if the ítiff Parcicles of che Air,
eicher pafled over che Mouch of
che lictle Tube CP j or elfe ftick-
inginit» like lítele Pieces of Wood
a-crofs ic, fupported che Colutnn
of incumbenc Air, fo as ic íhould
noc prefs upon che Water under
ir, with its ufual Weighr : Buc by
often repeaced Experiments, íc is
füund, Chat the Water will rife as
high in fmall Tubes, though che
grqfs Air be exhaufted. fíee The

throHgh the
longer

Exper, of tbe ^Academ. del Cimento,^- ‘Branch^
p, yyj Ic is evidenc iherefore, that
all chefe Pha:noinena*s are to be af-
cribed co Accraéiion. See the Notes
above on 6 ^.

1. They are deceiv*d) Ic is ma'»
nifeíl, from Calculación upon Me-
chanick Principies, thac all Qiie»
ftions abouc a perpetual M-Otion end
in chis. To tind out a Weighc hea-.
vier than ic felf, or an elaftick For^e
ftronger than ic íelf, Vv^bich is
abfurd.

150

RO H AU System Part I.

be not depreíTed lower chan uíual beneath the Level ot
the Water ia the VeíTeb it will not run out into the
Air, as it ordinarily does j whence we fee, that the Air
puíhes it back with greater Forcé than it has to come
out.

88.^cano»i 88. For a further Confirtnation of a Thing which has
ticen fufSciently proved, Imay add; that fo faris the Wa-
Jme oj the coming eafily out at the End of a’ fmall T ube,

^'i'- that fometimes it will be forced to enter and aícend into

it, when it was entirely without befóre : Which may be
' tried, by holding a very deán fmall Tube open at both
Ends perpendicular, and putting a Dropof Water upon
the exuernal Superficies, which may entirely ftop the Hole
at the lower End, when it is got down thither j for then
you will with pleafure fee the Tube filled in the lame
manner as if the End of it was dipped in a Veflel of
Water.

89. xvh'at 89. After what has been faid in the fbregoing Arricies,
tp Latife cf jj. £¡jpy jQ underíland what is the Cauíé of the F¡ltrí$
ttraumts. Chymlfts : Eor the Piece of Woollen Cloth

which they put upon. the Side of the Veffel, in fuch a
manner, as that one End of it ia dipped into th^ Liquor,
and the other End hangs down on the Outfide'*'íower in
the Air, refembles a bent Tube, inviihichthe Water runs
as in a Glaís-Tube; And it matters not, if this Cloth
Woollen Tube be full of Holes on allfTlides, for the Air
which moves round ir, prefles in the Water which en-
i deavours to come out at them, fo that it is like one con-
tinued Covering.

. 90 . That the 90. Sincc ourThoughts, orif you will, our Conjetures
concerning hard and li^iád Bodies are confirmed by fo
■Boities as many Experiments, I think it fuperfíuous to add any
^rfn Thing more. Wherefore Ifhallfinifh this Chapter, with on*
ñrraT" fcmarking two Things : The Firft is, That if Hardnefs
and Biquidtiefs confiíl in Reft and Motion, which have
their Dependence upon fometbing elle ; then thefe
Forma are not Subfiantial, but only ^alities or Modes
of Exiífence in the Bodies to which they belong. ,

Dr That having explaincd the Ñature of

bJardnefs and Softnefs, I have at the fame time explained
wherein Drjnefs and Moifl7iefs confift. This is evident,
jf we underíland the Word Dry and Moifl in the Senfe
of the Andents, who did not dillinguiíh them from hard
and liquid: As we may fee from henee, that fpeaking of
Moifl, they ufe the fame Greek Word as all Interpreters
¡render bumfl ojr liijuid jnd.iífcrently. It appe.ars further.

Chap. 23. 0} JN ATURAL Philosophy, 15E

thac I have explained what the Nature of Brynefs and
Moi¡ine[i is, according to that Senfe v/hich we now ufe
thoíéWordsin ; becaufe by Drj/, weunderftand thacwhich
will not wec any Thing ,■ and by Moi^, that which will
wet a Thing, which are two Properties which have been
fully and expreffiy handled above.

CHAP. XXIII.

0/ Heat and Coid.

T he SE Two Words have each of them two dif-

ferent Meanings : For Firít, by Heat and Coid, we aZclldMve
underftand lino f articular Senfatmis in us, which in foriie tm diffirmt
^eafure refemble thofe, which we caU Pain and Plea-
Ture ; fuch as we feel, when we touch Ice, or when
we go near a Fire. Secondly, by Heat and Coid, mk an-
derñand alfo the Power which Bodies have to raife the
foremeíMoned Senfations in us.

2. I think we cannot underftand what Heat and Coid,
in the former Senfe of the Words, is, but only by Expe- tlaiáefro. ■
riencej wherefor^vour Curiofity will be fatisfied, and onx fof^^ "cat
Pains imployed only iti enquiring what that Power confifts,
in, which certain Bodies have to warm us, and alfo what
the Power confifts in, which we obferve ocher Bodies have
to COol US.

3. Ariflotle fays, that Heat is that which colledts to- hm» Arí—
gether homogeneous Things, or Things of the fame Na-
ture, and diffipates heterogeneous Things, or Things of a and Coid,
different Nature j and Coid, he fays, is that which colledts ^

togecher, Things homogeneous and heterogeneous indif-
ferently. The common Inftances made ufe of to prove
this, areRVej by the Heat ofwl^h, a great many Parts
of Gold may be colleded irfto o* Mafs, or two ofmore .

Metals which are mixed together, may be feparated ;

And Ice, which by its Coldnffs, unites together. Water,

Stones, Wood, Straw, foas to compofe one Body of all
thefe together.

4. But it is to be obferved, that the Inftance here gi- 4, Thai Hm
ven, is fometiraes faulty ^ for if a Mafs, compofed ofGold,

Silver, and Copper, be put upon the Fireina Crucible, /liffJrmt

;ar them- ttatme, áS

íheir íjafttrt.

!t is not true, that thefe Metals wül always cl(
fdvss of each other, fo_as to be feparated an^

4

152

That the
yroperty of
jTire, ii ru-

co tic ¿i tose-
ther.

RO H AULT’s Syst-eia Partí.

their proper Order, one upon another, according to
their difFerent Weight. On the concrary, if feveral
diftiníl Pieces of Gold, Silver, and Copper be put toge-
ther into a Crucitle, the Pire will not fail to mix them
all together.

5 . ít is true, that if the Pire aífs a very long time up-
on a Mafs, compofed of Gold, Sitver and Copper ; the
ther to diff!- Süver and Copper will go all away in Smoakj and fo leave
patethan to ¡.jjg Qq]j aJone in the Crucible. But we ought not for
this Reafon to fay, that the Fire has a Property of col-
lecling Things together, becaufe this perhaps is only ac-
cidental that is to fay, by diíBpating the Pirft, which
refiñs its Porce leís, the Gold remains alone, or laft, be-
caufe ir refifts its Forcé more. In the fame manner, as
if Saw-Duft, and the Filings of Lead were mixed toge-
ther in a Píate, we can with our Mouths blow away the
Saw-Duft, and leave the Lead-Filings alone in the Píate.
For it is evident, that it is only the Refiftance of tha
Pieces of Gold, which is the Caufe of that Metal’s being
thús feparated from the Süver or Copper. Por if it be
ieft after this upon the Pire, it will continually diminiíh
by litcleand little, till it intirely vaniíhes, as Refiííers have
tried ,• and this is what they mean when they fay, there is
no Gold of 24 Carats, that is, none that can be refined
fo puré. ^ *

6 That At)- ¿ But if it vvas true, that Heat always colleded toge-
l°ja!dZhat ther homogeneous Things, and diffipated heterogeneous
HratmdCoid oiies, and that Coid colleíted together all fort of Bodies
do but not indifferently, this would indeed teach us txihat Heat and
» att.eyare. tcll US •what they are : But Ari-

Jlotle has been excufed in this, by íáying, that in defining
Heat and Cold as he has done, he did not fo much follow
his own Opinión, as that of others.

7. Wbnf the 7 - I don’t know wbether his Interpreters have hit right,
qpimmofhis whcn they pretend, that bis Opinión was^ that Heat, in
t¡oe Fire, for Inftance, * fomething in the Pire like that
Senfation which is raifS in íis, when we approach the
Fire. And fo likewife, that Coid in Ice, is fomething in
Ice very like that Senlation in us, which arifes from rouch-
ing it. ‘ Becaufe iq his 11. Book of the Sotil, Chap. xii.
after he had íliown tlrat Senfation is a Fajpon, he fays,
that the Moment any Senfation is rais’d in us, we be-
come like the Objedl that raifes it.

IcterpretSYs

Hfat and
Cold is.

Tíecaufe in hís II. ^ook) This
Píace is noc in r|iar Cbap^cer, but in
íbe vth Cbap. üf the fame Bjük, he

í TTCi^íi fA/Év ^ TO OÍ.yD{/jOÍOV,
^ á^otoy Ifiy,

8 . BqC

Chap. 23' Natural Philosophy. 153

8. But whecher Arifiotle were of this Opinión or no, 8. That they
thus much is certain, that they have no Proof of what
thcy affirm i fot it' is no Proof to fayas they do, that
the Pire cannot give that which it has not ,• becauíe
taking the Word give, in theSenfe here uíéd, there is no
doubt but that the Needle, when it pricks us, gives us
Pain, and yet there is no reafon to believe from henee,
that the Ñeedle has in it any Pain like- that which it
caufes in us.

5. Farther, the Heat of the Fire, and the Coid of 9- ThatitU
Ice being Properties or Qualities belonging to Bodies
which every one acknowledges to be inanimate, they '
cannot be like the Senfations which wefeel by theirMeans,
becauíe theíe Senfations belong to us as animare Greatures.

And becaufe the fame Thing may fometimes happen to

raife in us two difFerent Senfations at the íame time, it

will follow from their Opinión, that the fame Thing may

bj|fiot and coid at the fame time, which is impoffible ;

yet the Air which we breathe out of our Mouths, may

at the fame time feel hot or coid according as it is dif- '

ferentiy apclied to our Hands in blowing upon them.

10. By ‘lifledting upon this Experiment, which íhows 10. la «Sas
US, that the fame Air feels hot or coid, not only from its Uíat
being applied in a different manner to our Hands, but
alfo from the differ.ílit manner of making it come out
of our Mouths j it is eafy to conjeíture, that the Heat of
a Body conlifts in a peculiar ÍVÍotion of its Partióles. And
becaufe the nearer we put our Lips together^^ and make
the Air come out quicker and ftronger, the lefs we feel
the Heat ,- iience we conclude, that the Heat of a Body
íioes not confiíl in the direct Motion of its Parts. Now
whateyer is in Motion, either moves on direcUy, or elfe
has an unequal and irregular Motion, as it were about
its own Center,' from whence we may infer, that the
Air which comes out of our Mouth, befides that direét
Motion, by which the Whole of it is removed from one
Place to another, it has alio a great many of its Partióles
moved round with a circular Motion about their own
Centers ; By which means thofe which are applied to our
Hands, with this fort of Motion, exdte in us a kind of
Tickling And becauíe it is this kind of Motion which
taifes in us the Senfation of Heat, we ought alfo to con-
clude, that the Heat of Bodies coriffis in this Sort of Motms
their fmall Parts.

II. So

i54

ROH AU LT's System

Partí

1 1 . Tht
femJplance
ihcre is be-
twixt Hcat
and Fahu

II. So that what is iii the Objediis verydifíéreni: froni
the Senfation which it raifes. And this oughc not to bt
thoughc more ftrange, than the Difference chere is be.
twixt the Figure and Motion of a Needle, which prich
usj and ttie Pain which it caufes. For as it is evideni
from the Inftance of Pain, that the Soul being united to
the Body, it is the Appointment of Nature, that certain
Perccptions of the Soul íhouid follow from certain Mo
tions or Diviíions which the Needk caufes in the Body
So alfo we ought to think, that Nacure has appointed
that from that particular Manner in which our Body c
moved by the Fire, there íhouid arife a particular Percep

tíon j and this is what we cali Heat, taking it in the forme

12 . 777at'Bo^
dies vnay be-
come hoC) to
which it is
certain, no-
thitig has
happeneei but
Motion.

13. The I.
Mxam^le^

14. The II.
Mxamp>le,

ly. rAíIIL
Bxamplc,

j£,The IV.
Example»

17. TheV
JExampie»

Senfe of the Word.

12. This is confirmed by Experience, which teache
US, that raany Bodies are made capable of warming us
to which we cannot fuíped any Thing has happened boi
only Motion. It is to, no purpofe to inftance in t
all ; I Ihali content my felf with the
ampie.

13. And, Firñ, It is certain that when our Hands m
%’ery coid, we find by Experience, that if tÉíy be rttüi
a little ‘while together. we ihall feel a confiderabl
Heat.

14. Secondly, As was before obf^'ed. Lime having cá
lAater poured upo» it, though it was belore coid, will ac
quire fuch a Motion of its Parts, that they will be al
difunited in a íltort time, and by that Means will be
come capa&le of heating us in fuch a manner, that it v/1
be very painful to hold it in one’s Hand.

15. Rotten Dung, that is, fuch as diffipates it felf b
little and little, becomes fo hor, as to íerve inftead of
modérate Fire in many Chymical Operations. And Cby
miftry furnilhes us with many other Examples not fo coni
mon, which ought to be more known to the World
they are.

16. For Inftance, íf a few Vtlings ofBrafs be throw
into a large VeíTel in which is a little A^ua-fortis, it \vi
immediately raife fuch a Fermentation, that the Bott
will feem quite full, and at the íame time \vill be fo ho
that we cannot touch it witb.out being burnt.

17. Further, íf, as was before faid, Oil of Vitriol an
O// of Tartar be roixed together, though feparately nc
ther of them are combuftible, they will immediately a
quire an incredible Fermentation on a fudden, and at ti
fame time a very fenfible degree of Heat.

18.

Chap. 23. 0/ Natural Philosophy. 155

18. It is true, that in thefe Sort of Examples^ k may i8.r¿evi.
with fome Reafon be faid, there is fomething that we do

not throughly underftandj wherefore I fliall flay a little,
before I fay what the Caufe of thefe furprizing Motions
may be : To come therefore to fome more familiar In-
ftances j we obfervej that two hard Bodies rabbed againji
0!is another, do fo agítate the Parts of eachorher, as notonly
to buril US when we touch them^ but their Motion
wili increafe to fuch a Degree, as to fet each other on
Pire. Thus in very dry Weather, the TVheel and the 1

Axk-Tree of a Chariot, when ic goes very quick, and in
general, all Sorts of Engines which are made of Matter
that will burn, and which move very quick, are apt to
take Fire. Nothing is more common, than to fee a
Wimble grow hot in boring a Hole in a hard thick Piece
of Wood. So likewife, if we file or fijarp a Piece of Iron
or Steel, it will grow fo hot fometimesas toloíe its Tem-
per. And a Saw, which the W ood will not eafily yield
tdíacquires a very notable Heac. But nothing fooner
takes Fire than a fmall Piece of Flmt or of Steel, which
is ftruck oíF, and put into a violent Motion by ftriking
thefe two,%gainfl: each other. -Now in all thefe In-
ftances, there is nothing added to thefe Bodies but Mo-
tion.

19. All the Antier^ who have coníidered the greateíl '9- Ex-
Part of thefe Experiments, have alferted that Motion is IhTopMmof
the Principie of Heat ; which I acknowledge with them the hntUnti
to be true ; if by Motion they mean the Motion of the ‘"ocernitig
vihole Bodies, which is the Caufe of the two Bodies rub-

bing againft each other j but if by Motion they mean the
Motion of their infififible Parts, I think they have not faid
tnough ; For the Motion of thefe Parts, is the very Heat it
yfd thofe Bodies.

20. I fee no Objedtion that can be made againft thís : ao- F'hy ¿t
For when they objeót, in order to fhow, that Motion is

not > the Principie or Caufe of Heat, that a Ball out of »ry ^iiicks

1 Cannon which moves very quick, does not burn the f^smt greta

iVood which it enters into; or that a Musket Bullet does

tiot burn the Wood which it penetrares, though it Be very

Jry ; this contradióts the Opinión of thofe only who pre-

tend that Heat confifts in the Swiftnefs of the Motion of

ill forts of Bodies how grofs foever. But this Qbjection

makes nothing againft us, who aíBrm, that Heat confifts

¡n the difíerent and violent Agitation of the infenfible

J. The Priactpií or Caufe of the Heat it felt

Parts

¡¡56

■ RO H A U LT’ s System Part I,

Parts of Bodies. Buc when a greac Bullet moves very
Guickj its Parts may be ac reft wich refped to each other,
and cherefore ic is no wonder thac rhey don’c burn the
Bodies which they touch.

ai. Whythe ¿I. If we refled upon what has beeii faid, we ftall
Whnigrtw wonder^ that che Bands of Iron which are about

and not a Wheel do not grow hot as it does in the Middle j fot
theFeiiíws. though they defcribe larger Spaces by their Motion, yet
notwithftanding chis, their Parts are not agitated with rc-
Iped to each other, as thofe in the Middle are, which con-
tinually rub againftthe Axle-Tree.

22. Whya 22. We may very eafily anfwera great many Queílions
^hen%ed‘^^ which may be put to us by thole whb will not allow, that
growUt’but the Forih of a hot Body confiíls only in the Motion of its
not the File, fmalleíl Parts ; Thus when they ask, how it is poffibk)

thac when a Piece of Iron fixed in a Vice, is filed, the Irm
grows confiderably hot, but the Fi/e which moves upon it
is fcarce warm at all; Ic is eafy to anfwer, that the Pm
of the File moving upon the Iron, and continually grating
itj not only with its own Parts, but alfo with lome of
the Parts of the Iron which it has rubbcd olí, and which
r.emain fometime betweenits Teeth, muíl n^eíTarily ex-
cite a very great Agitación of the Parts of the Iron
which is filed, and confequently heat it very fenfibly. But
this is not the Cafe of the File,- f(jt¡ithough its Parts are
grated as much as thofe of the Iron, yet becaufe it is
longer, the fame Teeth do not twice together touch the
Body which it grates, buc there is always fome fmall
diftance of Time, between the two Rubs of the Parts
of the File j during which time, that Place which may
have begun to acquire fome fmall Heat, may lofe it
again.

ít-.whyJnn 23- There a,re fo rtiany Things to be confidered in this
iehm it is Experimenr, that a fmall DifFerence alters all the Circuin-
^uJ'tTan ftances. Whence ic is, that a Piece of Copper or Lead,
other Metals, when it is filed, oughc not to grow fo hot as Iron, both
becaufe Copper and Lead are not fo ftifL and becaufe it
is eafier to feparate their Parts than the Parts of Iron ;
fo thac the File being never applied twice together to
the fame Pare of the Body which ic grates, it cannot
íhake its Particles fo much : And this is lo true, that if
vie. try ta file a Piece of Copper, wich an oíd worn File,
which will IlTiave offbut a lirtie at a time, the Heat wiU
be as great as that produced ia the Iron.

5,4. New

Chap. 23- of Natural Philosophy, 157

24. Now if any one asksj why, in fawing a Plank of 24. Whj e
Wood, the Saiu grovvs ho: and not the Wood: I anfwer,
that the Píate of the Saw, íticking in the Slit of the
Wood, and being rubbed’againft each Side, the Parts of
it muíl be íeníibíy fhaken : Whereas ic- is evident, that
the Plank ought not to grow hot in the Place againít which
the Teeth of the Saw go, for the Reafon jufb now given,

D/a. becaufe it cuts the Parts oíF,- neither ought it to
grow hot on the Sides, efpecially if the Wood be eaíy to
faw, becaufe the Saw advances further and furtherinto the
Slit, and fo does fcarce twice together touch the fame Part
of the Wood.

ay. It is true, that if the Wood be very hard, and dif- ffo» tóe
ficult to faw, and if the Saw fticks in the Slit which it
makes, the Plank will then become pretty hot ; but we Zu.
íhall not be able to perceive it by our Touch, becaufe
the Parts of the Wood being large, lofe their Motion in
a^oment, and it will take fome time to pulí out thé
and to open the Slit Ib wide as to put our Hand in
to feel. But though we cannot perceive it by our Touch,
we may fee it with our Eyes ^ for the Places againít
which th^Saw for fome time grated look burnt, as if
they had been in the Pire. And it happened fome
time ago, that defignedly fawing a Piece of hard Wood,
fixed in a Vice, in.j^Smith’s Shop, with a Saw which
ftuck in the Slit it made, I at firíl perceived a Smell W

like burnt Wood, and continuing to faw the Wood with
greater Forcé, feveral Sparks carne out of it.

26. The Experiment which feems to be the moft con-
tfary to the Principie we have laid down, is, that if we ^
drive with a Hammer a large Nail into a piece of hard o'fWoodZTth
Wood, we íhall not find it grow warm while it is dri-
ving in, but after it isin, and the Hammer does nothing
elfe but beat the Head flat, then it will begin to acquire
fome Heat ; Yet is there nothing in this, but what per-
feftly agrees with our Notion of Heat. For as we make
it to conGft wholly in the Agitation of the fmall Parts
of the Body; ic is certain, that the Nail ought not to
grow hot, when it is moved all together in entring into
the Piece of Wood ; but that ic ought then ,to begin to
grow hot, when it ceaíes to move fo, and its Head begins
to be made flat ; for it is then only that the fmall Parts
begin to be in Motion, and acquire an Agitación fuffici-
ent to Heat. And indeed, when the Head of a Nail is
made flat, all that is done, is, that there are by that Means
fcvver Parts placed one upo^nother, and more by each

other’s

a6. mya
Nail driven
into a Fiece

158

RO H AULT’s System Partí.

other’s Sides; which cannot be, but by the Motion
and Agitación of thefe Parts, which by their beating
againft each other, caufe thac trembling in which Heac
coníiñs.

27 . That 27. Having thus endeavoured to anfwer che Objections
^íh7verfho‘ madc againft us ; we come now to draw

fome Confquences from what we have laid down ,• be-
caufe if thefe agree wíth Experience, they will help to
confifm US in chis, that we are noc far from che Truth. In
the firft Place chen, lee us coníider, that feeing Heat con-
íifts in a certain Motion, or a certain Agitación of the
fmall Parts of a Body ^ it is certain, that the more the
Parts of the Body are thus moved or agitated, the greater
will the Heac be. Now it is evident, that i Víame is
more agitated than any other Body which comes under
our Senfes. For, forExample, it is this violent Agitación
af the Parts of the VVood which nourifli the Fíame,
that makes the greaceft Pare of them fly away, and thac m
all che Wood that can be burnc in a Day, fo very lictle re-
mains in Afhes j which we do noc find in che foremen-
tioned Inñances, where chere is only a modt|;^te trem-
bling of the Parts of the Bodies which is noc fuíBcient
to difunite them entirely. And this is the Reafon why
F/ame oughc to be the hotteft Thin» inthe World, as e-
0 very Body knows it is. V

¿s. Hm a 28. However, this muft be underftood with fome Re-
ftridfion, that is, if they agree in all other Particulars;
for it is noc inconfiftent herewich, that there íhould be
Fíame may fome Bodies hotcet, and more capable of heating than
yetbehotter. pig^le, if they coníift of mote íblid Parcicles, anc
confequently fuch as are more capable of Agitación;
p wherefore Ira-a, tho’ it be not red hot, will burn more,

if we touch it, than the Fíame of Síraw, or Spirit of Wim
will do.

29 . why^ 29. The DifFerence that there is betwixt the GroíTnefs
Sea-Coai mil gf ^ jjg Particlgs íoto which the Bodies thac are burnC are
X” o- refolved, is the Caufe of fo much Difference in the Flama,
íher. Thus, Oak being more folid than Straw, but noc ib fo>-
lid as Sea-Coal ; their Flames are alio pro^orcionably more
or lefs burning or ftrong one than another ; And the Ufe
that Smiths make of them, according as they have oc-
calion, fhows plainly, that Sea-Coal, a£ts more ftrongr
ly than all other, becaufe when they would heat a

I. Fíame is more agitated) Con- I Fire, See Fart the Illd. and the
cerning the Nature of Fíame and ix. Chaf, aith the Notes.

Chap. 23. Natural Philosophy; 15^

Piece of Iron very much, they prefer this Goal to all
others.

30. When a Body melts, and liquifies, as I may cali 30. Hb»
ir, by licde and little into Fíame, it is impoffible buc that

the Particles which ílip and rub one againft another, muft a?,d dimi- '

be diminillied and broken into a thoufand Pieces, and ib

make a very fine Daft, which, that it may continué to

move with that violent Agitation which it has acquired,

gets off from that Maís of which it was before a Part,

and flies into the Air 3 which is what we cali exha-

ling or evaporating ; And iqence it is, that the Fire

has the property of Hhnhitfimg all Bodies which it ads

upon.

31. This being allow'ed, there is no Difficulty in re-ms'.
folving that Queftion commonly asked, viz. How it i^cuymdjií^
poffible that Heat íliould produce at the fame time two tais Wsix-
feemingly contrary Effeds : Such as hardning of'Vlay,

a|d foftnmg of Wax. In order to this, we need only ob-
ÍOTe, that Clay is compofed of two Things that are ve-
ry diflferent from each other, viz. Earth and Water ¿ the
Latter of which may very eafily be evaporated, before
the Partic'^s of the Former are confiderably fhaken 3 and
fince the Clay is foft for no other Reafon, but becaule the
Particles of the Water are in fome fort of Agitation, a-
mongft the Partidi^i of the Earth, to which they be-
long3 it muíl needs be, that when the Water is all eva-
porated, and the Particles of the Earth remain alone,
they will reft againíl each other, by their own Weight,
and fo by that meanscompofe a hardBody. Onthecon-^
trary, the Farts of Wax are pretty near equal j fo that
the grofler Particles are agitated before any confiderable
Quantity of thefmaller ones can fly away. And therefore
all the Particles of a Piece of Wax being a little in
Motion at the fame time, compofe together a foft
Body.

33. It may be obferved alfo, that the Heat muft be That the
but modérate, ^harde?z Bodies: For if it be very vio-
lent, it will’make th&m lifdd. And thus we fee, that ^reatJÓhaT-
Flátne melts not only Metals, but alfo Afires, Sand, Stones, ‘BodUs.
and FlintSj of a Compofition of which all Sorts of Glafs
are made.

33. From the different Degrees óf Heat, and the va-
rious Texture of the Parts of which a Body is compo-
fed, we may conclude, that very difFerent EfFeds will be
produced : For firft 3 If a Body, <whofe Farticles are very

hot, whateyer the
Figure

'jr6o ROHAULT’s System Partí

Figure of thefe Particles be, fo tbey be not exaflly round,
when they are agitated or turned round their Cencers, theit
angular Points, or the Parts which are moíl diílant fro®
the Center, muft neceflarily meec one another, and turn
one another ouc of the Way ,• whence it follows, thac
the Heat will caufe a Rarefadion in this Body, as we fee
in Milk, and all ocher Liquors j and alio in moft hard
Bodies, in which few or none of their Particles fly olf
when they are hot ; Thus red-hot Iron is fomething big.
ger than when it is coid.

54. Hm it 34. But if the Particles of a Body be very fmooth, ani
Motion-) and yet are Jo flaced nuith refeñ
to each other, as fcarce to touch osie another ¡ fo that tk
^ompofition is very rare j a very little Heat coming upen
itj and Ihaking the Particles, may caufe them to approacli
nearer one another, and the vJbole Body may be by this
means condenfed. And thus we experience, that Heai
■ when it melts Snow, reduces it into a lels compafs.

5J-. vi^hy 35. And becaufe the Particles of almoft all liquid

every Moment bend themfelves, or fome way
freexing”¡r alter their Figure, in order whereunto, they muíl be trio-
rarer than yed with fufficicnt Fotce j therefore if the Hg’ t, or chai
which forces them to move, or fo agitares them as to
make them Liquid as ufual, does almoft wholly ceafe,
all that the Particles cando, with thatjittle which they have
remaining, will be to raove themlé»/es without bending
fo much as is neceíTary to join them togerher : And
tiren the Liquor will be rarified a litde, and after it is fo
‘ rarified, the Addition of the lesft degree of Heat, will
caufe its Parts to approach nearer one another again. Thus
Water is a little rarified befare it freezes, and is conden-
fed again by the leaft Heat that can be. But becaufe
fome Skill and Pains is requifite to prove this by Experi-
ence ,• I will fet down the Means 1 ihade ufe of to make
it appear fenfibly.

35. ^nEx- 36. I caufed a Glafs Veffel to be made like that in tlie
to Figure, the largeft Mouth of which is at A, and the othet
fZlr^when B, the End of the fmall Tube BC, 'wlbichds very fien-
itiscxtrime- dcr ; I poured Water into the Hole A, ’till the VefTel
rififd' ” confequently ’till it arofe up to D, in the
Tafa. III. fmall Tube, tiren I ftopped.up the Mouth A clofe wiiii
fig. ip. foft Wax, and a Hog’s-Bladder tied on : Having thus
prepared it, if the Heat of the Air be fo diminiílred,

thri

Chap. 23. of Natural Philosophv. ' 'ibi

thac the Water be very riear freezing, *.ic will fwell,
and rile up to the Mouth B, where ic will fometimes run
over alittle: Then^ if we put our Hands or any other
^ Thing that is warm, to the Velíel, we íhall fee the Wa*

^ ter condenfe it felf, and finlc in the fmall Tube almoft
I down to the Bottotn C. It is true indeedj that if we
' continué to heat the VeíTelj the Water contained in it,
will begin to dilate itfelf again, the Reafon of which, is

■ thac which I have now given.

37 . Becaufe we can move our felves wich greater Eafe
in the Air chan in the Water, chis is a Proof that the HeaT,‘mafb^

. Parts of the Air are much finer than thofe of Water, deurmmedby
. Wherefore the leafl: Heat thac can be, muft dilate the
: Air; and confequencly, The ^antity of the KarefaBionof Mr.
the jíir, iiStll very exañly ¡how the ffiantity of Heat here
071 the Earth^ that is to fay, we can judge that it is hotter,
one Day of che Year than another, by obferving in which
áf thefe two Days, the Rarefadion of the Air is greateft.

™ 38 . Now in order to malee chis Rarefadion feníible,

there has been invented in our Days an Inftrumenc called Thermlmetcu

■ a Thermometer, pretty like that in the Figure : DF is a Tab. iv.
very fmH Tube of Glafs abouC two Fooc long. like a

Neck belonging to the Boctle A, which is Glaís alfo,
and about as big as a Tennis-Ball. The lower End is bent
^ and made large, íqtós to forra anocher Bótele marked F,
which needs not befo big as the Bótele A, and has a fmall
1 Hole made in ic ac B.

: 39 . The Ther7no77ieter is at 6 rft entirely empty, that is, ^39-

I full of Air only, pare of which is forced out, by heating prejlr!n¿»íi

■ the Boctle A, at the lame Time that the other Bótele F is thevfeofthe
. dipped into a Velíel of Aquafortis, tindured of a Green ^*«r»>omefír.

Colour, by diíTolving a Piece of Copper in it. We
choofe Aquafortis rather than common Water, becaufe
; it is not ío fubjed to freeze, and does not fo eafily eva-
; porate. As che Air remaining in the Thermo7/3eter grows
; cooh it has not Forcé enough to preferve that Bulk which
ic had before, and fo is obliged to retire up into the Glaís,
and leave Room for the Aqua-fortis, which by its own

* It will fxoelí) Becauíe íes Parts
are made íHfF, by the Mixture of
i^itrous Particles, and of other Sales,
[Sce the Notes otí ^rt, f4,) Howe-
veric muft be acknowledged, thac
fbmething oughc to be allowed for che
ContraSión of che Giafs. For as
Heat by encreafing che Mocion of
*he Parts, dilaces and excends Glafs
and oifaer Bodiesj fo coid by

the Motion of their Parts contraías
and condenfes Glafs and other Bodies,
See the Exferiments of the ^cad. del
Cimento, p. 109, The WateC

cherefore a licele beforeicfreezes, ri-
fes in che Tube CB, partly becaufe it
is a IlccIe rarified, and partly becaufe
che Glafs AG Í5 a liule condenfed by
che áold»

Tab. III.
Hg. JO.

5^2

ROH AUL’T’s System Partí,

Tab. IV.
Fig. I.

.lü . Tht R eít-
fuTi o/ this

Í 7 >.

4f. Thnt this
7'hermomcUr
do es nnt cx-
a7ly d'ijiin-
^nilh all i he
JPtfcrrnces of
the Heat*

42 , De'’
firiptim nf
tí'fíoíherTher-
Xíiumetef.

T ab. ÍVh
F ig. 2 .

Weight, aílifted by that of theexternal Air:, gets intothe
Bottie F:, and from thence rifes up in the Tube towards
C. After this, the Inñrument is taken ouf of the Veffel
in which ic was dipped, and without doing any Thing more
than fixing it in a Wooden Frame, marked vvith feveral
Diviüonsj itfliows how much hocterit is at one time than
another.

40. For the more the Green Liquor is forced to de-
feend by the Rarefadtion of the Air in the upper Part of
the Tube, the hotter it is in the Place where the Ther-
mometer is fixed : And on the contrary, it is a Sign of
gvearer Coid, when this Liquor rifes higher, becaufe this
íhows that the fame Air has not Forcé fuíScient to pre-
ferve itsBulk, but isobliged to give way to the Aqna-joi-
tis, which the Weight of the external Air that preíTes
upon the Hole B, continually forces to rife up as high as
it can in the Tube DF.

41. However, we muft take care not to be deceivÉ
in the Judgement we make of the Heat, by barely loof-
ing on the Ihermometer ^ becaufe the Weight of the Air
being not álways equal, it may be, that the Air. jvill prefs
more upon the Liquor contained in the Bottie PJ' at fome
Times than at others, and coníéquently forcé it to rife
higher in the Tube FD, and may occaíion us to think
that it is colder than it was befpre When perhaps the
Keat of the Air was neither greater ñor lefs.

42. This occafioned the making another Sort of Ther-
rnometer not long íince, which has but one Bottie of G'lafs
only, and has a long Hender Neck as is here reprefented.
At the Hole A is put in as much Spirits of Wine as
will fill the Bottie quite full, and the Neck alfo as high as
the Place marked B, and then putting the End A into thé
Fíame of acommon Lamp fuch as Workmen ufe, ñopup
the Mouth there, and then the Thermometer is finifhed.

43. vvhy 43. When the Heat of the Air increafes, the Spirits of
t^ilate and rife above B, and fo forcé the Air in
^ir in this the Part of the Neck BA to condenfe. Which it

Therrmimftcr
Tr.b. IV.
Fig. 2.

■tnty eaíily do, becaufe when it was inclofed here, it
was very much dilated by the Fíame which melted the
Glafs, in order to ftop the Hole A. On the contrary,
v/hen the Weather grows ,coId, the Spirits of Wine
contradi into a lefs compafs, and defeend below the
Place marked B, and permit the Air to extend it felf be-
yond its Limits.
v/becher it
the

Chap. 23. rOf N ATURAL P HILOSOPHY.

. 1 %'

the Inequality of the Weight of the Air, becauíé it
cannot gec in, to make any AJteration in our Obferva-
tions.

4¿j.. Though the Fauk in the foregoing ThermonKtef is
retnedied in this, yet has this another of as ill Confe-
quence, viz. That becauíé the Spirits of Wine dilate
andcondenfe but very ílowly,we cannot foon enough per-
ceivethe Alteration that is made in the Hearor Coldneís of
the Air. And diere is another Fault ftill, (if it be not
nlade larger than they ufually ate ) which is, that the Spi-
rics of W inejbeing not capable ofa yery great Rarefaction,
its Rifing and Falling inthe Neck of the Botde will not be
of fo great Length, as to diftinguifh the fmall Changos
that happen in the Heat of the Air. But one Remedy
of this, is, as I faid, 1 to make the Theimoinetcr very large.

I have one in v/hich the DifFerence betwixt the greateft
and leafl: Height of the Spirits of Wine is above three

f oot.

45. After what has been faid concerning Heat, there
remains nothing more to be explained, but that which hot by ha-ving
we experience in UmSi lohen either Water is poured upan pi»r-
itt or i^ put into Water : And this may ferve to explain
why other hard Bodies grow hot as foon as certain Li-
quors enter into their Pores. In order to our Satiafaéti-
011 in this Mattei\;i^e peed only confider, that the Stone
of which Lime is made, has fo very fmall Pores that the
Water can fcarce enter into them j but after it is put
into the Kiln, the Fire which penetrates ih carries away
fome of the interoal Particles, and by that Means enlar-
ges the Pofes fo much, that afterwards the Particles of
the Water can eafily enter, being only furrounded by
the 2. Matter of the firfl: Element •. Wherefore being freed
from the Matter of the Second Element, when they en-
ter into the Pores, they can eafily acquire all the Forcé
of the Firíl Element in which they fwim ; fo that mo-
ving very quick, and being alió pretty grofs, they/ have
Forcé futScient to difunite the Parts of the Lime, and
to carry the fmall Duft of it along with them ; And it
is principally in the Agitation of this Duft that the Heat of
the Lime confifts.

I To V 7 ake the Thermometer) This
Inconvenience may be remedied by
feending che Neck of che Thermome-
cer inco a Spirali forby rhac means
■the Spirits of Wine will rife eafier

and quicker, and che Difference of
che Degrees of Heac may be more ea-
fily obfervM.

2. Matter oftke Firft Element) See
chq No.iesbelow on Are. 48.

45. There

T^4 ROU AULt* i System Partí.

46. Hbw a There is no need of wetting Hay in order to have
^Ha *'■ íufficientj if it be heaped up

^j^^í-íwí jf greeíí^ for every Spire of Grafs contains in

ic felf enough of the Moifture which it fucks out of
the Earchj the Partióles of which go and come out of
one Spire into another, and fwim at firft in the Mat-
ter of the Firft and Second Element, where confequent-
ly they have only the Velocity of the Second Element,
But afterwards when the Grafe grows dry, their Fibres
íhrink, and their Pores grow fo fmalh that the earthy
Juice which runs out of one into another^ fwims in the
Matter of the Firft Element only, whofe Velocity ir
then obeysj and fo has a Forcé fufficient to move
the groíTer Parts ofthe Hay^ and to heat them by that
Meaos.

ny.WhyHay 47. I faid expreflly^ that the Hay muft be heaped;
7 '’^" ‘‘Jj Partióles of the earthy Juice which come out

naiTíle. of one Spire of Graís may enter into another with ^
their Morion ; becaulb if the Hay be fcattered in the
Meadow, the Juice which comes out of the Spires of
GrafS:, is diffipated in the Air and does not enterjgain into
others:, to caufe that Agitation which is necelíafy to pro-
duce Heat.

48 . Hom 48. As to the Heat which arifes from the Mixture of
ítuo Liqmrs different Liquors, we need onlyíbnagine i that their
Particles are of fuch a Figure, that they can more clofely
«i¡xed to¿í. unite when they are mixed together, than when they are

ther.

I. That their Farticles) Since there
ís no fuch ching as chis FirJ¿ Element,
byall chefe Experimems, it appears,
that in FermcritationSj the Farticles
of ‘Bodiesj xohich aímoji rej}, are put
into neto Motions bj a very potent
Principie (namely Attraflion) xohich
a^s npm them only xohen they ap^
proach one another j and cattfes them
to meet and claj ¡7 with grcat Vió-
leme^ and to grow hot voith the Moti-
on. Newü. Opc. pag. 3) y. But
becauíé Heat does not confiít in eve-
ry Motion, buc in a peculiar Mo-
ción (and of cercain Parcicles per-
haps) üt the Imall Particles of all Bo-
dies 5 if the Fermentación or E-
bulütion arifes from chs Mixture of
íuch Sort of Saks as produce Coid.
(Sec the Notes on jirt. 5*4 beloro)
che Fermentación may not only be
attended with no Hear, bucwich a
ítínüble Coid. Thus Salt-petre mix-
ed with Splrit of Vitriol or ocher
fcid Spirits i alío volaiile Salí of JJ.

riñe with diflilled Vinegar or SpU
rit of Vitriol', alíb Sal Armoniac^LtÁ
Corroflve Sublímate reduced to a
Powder feparatelyj and then mixed
together i ‘údiftilled Vinegarhepom-
ed upon them, they will be very
coid during che Fermentación. (See
the Philofoph. Tranfaélions N° m 174 *)
Alíb Sal Armoniac mixed with a
double Quantíty of Oil of Vitriol
will bubble up and íwell very much,
and yec che Liquor ac the fame time
feel very coid. bee the Exper, of
the Acad. del Cimento, p, 153» Nay
furcherj from the Motion of fome
Saks which are naturally in all Wa-
ter, it ís, that Water it felf inclo-
fedÍQ a Glafs, and put into a larger
Vefl'el ftill of Water, if red-hoE
Coals be thrown into the Water in
chis larger Veffel, will firft grow
coid (as appears by applying >^4.
Thermomecer to it) before it re«
ceives che Heat communicaced by
¿Vater which furrounds it.

íeparatCj

Chap. 23 ' Natural Philosophy. 165

feparate j and wben they are fo mixed, they fwdm in the
Matter of the firíl Element: onlyj ac leaft, during that
lítele Time we fee them ferment : Which is confirmad
from henee, that after the Fermentation ceafes, we find
many Particles united together, and that they compofe
a great many fmall hard tíodies.

49. Having thus explained the Form ofa íiot Bodj/, ¡I’

will be eafy to determine that of a coli Body, which is NatarTqf‘ ‘
the direót contrary ; For if we confider^ that CoU extin-
guiflies, or rather diminiíhes Heat, there will be no
I)oubt, but that thofe are told Bodks, which caufe that
particular Motion in which Heat confiíls to ceaíe :

Now we know that this Property belongs to three
Sorts of Bodies : Firft, to fuch as have their Particles
at Reíl with refped; to each other. Secondly, to fuch
whofe Particles may be in fome Agitation, but lels than
thofe of the hot Body to which they are appiied ,• and

Í L,afl:ly, Such w'hofe Particles may be fufficiently agita-
ed with a Motion proper to excite in us the Senfati-
on of Heat, but is attended with a diíFerent Determma-
tion which changes and flops the Motion which the
Parts c!t our Body are in, and therefore cool it. The
whole Difficulty therefore is, whether Coid confiíls in
one of thefe Modes only, or in each of the Three.

50. Now lince there are Three Sorts of coid Bodies,
we may affirm, i'i^t Coid confiíls in each of thefe three ‘iréíscmif
Modes. For, Firft, The Cold-which is common to all íM "BíHu.
hard Bodies, cannot confift in any Thing but wbat is
common to them all, miz,, in the Reíl of their Particles:

Further, the CoU which we feel in Summer-time, when
we go into the Water, efpecially when we are up to
the Middle, arifes from henee, that the Particles of the
Water having leís Motion, than our Bodies have in all
thofe Parts which are near the Heart, they receive Ibme
Motion from us, and at the íáme time we lofe it. And
of this we have a very convincing Proof, becaufe the
fame Water feels many times warm when we dip our.

Hands into it, becaufe they are not fo hot as our Breaíl.

Laílly, It is evident, that the Breath which comes ouc
of our Mouths, when we contradi our Lips, or the
Air which we put into Motion with a Fan, in the
Heat of Summer, ought to cool us ; if we confider that
the diredl Motion of them diminiíhes or alters a litth
the Determination and Agitation of that Motion which
is in the Parí of the Body where we feel it cool.

i66

RO H AULT’s S ystem

Part I.

52 . yy^hy

Jome “Bodies
are colder
iban oihers.

jt. vi'hya 51. For a Confirmación of chis, we may obferve, thac
^^mit^íooh Bodies cannot make any Akeration in the Motion of
amthtr, hot Bodicsj wichouc as much alcering that Mode in which
«anís itfeif. j-heir Coldtiefs conlifts ; that is, a colá Body can-mt mi
another, •vúithout groiumg v^arm it felf, and fo we íind by
Experience.

52. We may obferve further, that the more Particles
a coid Body has at Reft, the more thofe of a hot Body
to which they are applied, ought to íofe of their Moti-
onj in order to communicate fome of their Heat to the
other. Thus Marble having more Particles at reft than
Wood which has more Pores, and is full of a Liquid
Matter which is in continual Motion, ought to feei cold-
er than Wood.

y-, Whytht Thís alfo may ferve to explain to us, nnhy the Ak

'uidxTdyts Marble-, or other Bodtes, •which have •very
colder than in Jmall Pores, ought not to he quite fo •Warm, or ought to be
other Places. ^ l¡f-(lg cooler, than that •which is in Places •wherefuch BodieÉ.

are not. For the groffer Parts of the Firft and Second E-
lement, which cannot enter into the fmall Pores of thefe
Bodies, muflí neceíFarily be refleífed back frotn^. them
and for the móít part there is only the moflí fuoLil Rdat-
ter about them, which is ready to enter into them, or
which cannot but come out of them ,• and confequently
this is not able to agitare the gro^., Particles of the
Air, which are proper to raife in us the Senfation of
Heat.

54. When I fay that Bodies which have more Parti-
cles at reflij ought to feel colder than others which have
fewer, I íuppoíe that the Particles of each of thefe Bo-
dies are equally fufceptible of Motion •, for if we fup-
pofe that the Particles of a Body are very eafily to be
put in Motion, and to loíé their Refli •, this Body, though
very porous, ought much racher to receive wichin it felf
the Agitation of a hot Body, and by that meaos cool it,
than another Body which has fewer Pores and more Parts
at refli, but fuch as are not fo eafie to be moved. And
henee it is, that when we tOuch Ssto-w, which is very
rare, it cools us much mere than when we touch Mar-
ble, whofe Particles are much lefs capable of being put
into Motion. *

55. The

VVhy

Snow feeis
colder than
MarbUt

¥, \t ís rauch more probable
efeat Coid (which is not merely
comparativíf as thac of Jtmply Hard
Oí Liijuid Bodies is i ¿uc pro-

duces real Effe£ls, fíich as Free-
zing. Breaktng in Pioces, Rareía*-
¿bions ó'c.) is owíng’ico íome Par-
-of jVí'íre and orber Sa/if
which

Cliap. 23 '. of Natürál PtíiLo'sopíiY. i6^

The Nature of Heat and Coid being fiich as I s^-Umbuth
hiv!-: uow dcfcribed, if yon cali to mind what was
•beíore faid concerning the Form of moiíl or liquid Bo-
diesj ic will be eafy to underñand how Heat and Coid,
which are dircí 3 : contrary Qualicies, may yet, though by
quite different and oppofite Ways^ produce one and thi
Jme Efi¿i, viz. Drjing orHardizing: As we experience in
chis , that the fame Things, as Cky, for Inílance^ are made
así/rvby the Coid in the Wintetj as they are by the great-
eíl Heat in the Sumtner : In order to underñand the Rea-
fon hereof, we need only confider, that the Parts of moift
or liquid Bodiesj fuchas Water, loíe all their Motion when
it is very coid ; wherefore fince fuch Bodies by this
Means acquire the Form of hard or dry Bodies, it is not
at all furprizing, that Cky which is compoíéd of Water
and Earth, iFould grow hard and dry, when the Wea-
ther is very coid j íéeing the Water alone, to which all
ihe Softnels of the Clay is owing, freezes and grows
"ard. On the contrary, Heat caufing the Parts of the
Water, by, whofe Means the Matter of the Firñ and
Seconc^íement kopt the terreñrial Parts of the Clay
in fomPfort of Motion, to evapórate ; theíe terreñrial
Particles, by their own Gravity, will bé at reft with refpedt
to each other, and by that Means compofe a dry or hard
Body.

5¿. Henee we may alfo fee the Reafon of a Maxim r<s. my
founded upon a Multitude of Experiments, viz,. That
Heat and Moijlurs are Principies of Corruption. For a Trimipiesof
Body is corrupted when there is a very remarkable Change
in it, which doubtlefs may be efFeiñed by fuch a Mo-
tion as this. Now thefe two Qualities confift in this
Motion.

57. On the contrary, by Reft, the Parts of Bodies are sr.tVbyCoid
kept in the íáme Situation, and Coid caufes them to be at

Reft ; wherefore we may lay this down for a Maxim, That
Coid hinders Corruption.

58. However we muft not affiriii this to be a general
Maxim. For if a Body has Pores large enough to contain f

a good deal of Liquor, and thefe Pores be filled with smss.
Water ; becaufe Water cannot freeze without dilating
it felf, it may fo happen, that in freezing it may break

which are of certain Figures pro-
per co excite that Senlationj and
to produce thofe Effedts. And henee
it.is,' thac/\S*3/ Armaniac or Salt-
F(tre, or '^alt of Urhe, and ma-

ny other Vx/latile ^ikalixdti Salís,
make the Water with which they
are mixedvery coid, See above on
Are. 48 •

the

4

i68

System Partí.

the Plfints»

the Bodyj which containsitj in Pieces. And thuswefee
thac fofc Scones, which are expoíed to the Froíl, crumble
and arereduced almoft to Powder, before the Water which i
they have fucked in, can get out.
yj. xvhy This perhaps ís che Reafon of whac is íáid by fotne

f»ifo'p/a"ts Antients, That a hard and fenetrating Frojr has a

' Power of Burning. However, it very ofcen happens, that
we afcribe that Effedt to Froftj of which it is only a
very diftanc Caufe, and which is immediately produ-
ced by Heat. Fot Exatnple, when we fay^ that Froft
corrupta Fruits and the Buds of Planes, we oughc rather
to fay • thac the Heat corrupta them whilft the Frofl is
diíTolving, becaufe it cannot get into the Pores of the
frozen Fruits, ñor make the internal Parts fo foft as
they were before they were frozen, without having firít
intirely deñroyed the Connexion and Order of the other
PartSj ñor confequently without having alteredthe whole
Compoíition of t):e Parts.

6o m CoM Proof of chis we may obferve^ that it is the e:

d/esmt hlrt treme Parts of the Planta, which always con'ain in them
“/ more Moiñure than the other Parts, that are almoft the
only ones Corrupted by the Coid, and alfo rbac fte Coid
does not hurt them till after they are budded j for before
they bud, the Coid does not hurt them ; for which we
can give ^ no other Reafon buc chis, thac Plants beíore
they put forth their Buds, are not fo ftñ of watry Juicos,
and their Pores are large enough to fuffer the fubtil Mac-
ter, to put thofe Parts which may have loft their Mo-
tion iijto Motion again, without neceíTarily deftroying the
Connexion of thofe it firft acta upon, and which are
more externa!, before it comes to apply it felf to the o-
ther w'hich are more internal.

6i. For a Confirmation of the Truth of the foregoing
Art. we may add, thac in Northern Countries, where the
Coid is fogreat, that a Man cannot go into the Air wdth-
out running the hazard of having the extreme Parts of
his Body frozen ; if their Nofis or Fingers be frozesz, they
do not lo fe them:, if keep front the Fire, and ntb them
witb handfuls of S»ow,

A Con^
ñrmation of

I, Tlmt the Hcaf corrupts them)
However for the mofi: parcj the Par-
tíclesof the Juice being dilated and
rnade ílifF by che Coid, bréale in
Píecesj and fpoil che. tender Parts
obferved by ^ír. Le

Olere in his Phyfcks, Hook V. Chap,
xiíi. Sííí?, Though thi^ DefeiS
does not appear till che íüllqwing
Heac (hows it.

2. No other RcAfon'^ Sce the Notes
OH the foregoing Arto

Chap. 24 - ®/ Natural Philosophy, 169

62- Having thus explained the Four principal Qualities ^hat tU
thatcome under the Senfe of 'Feeling, toz., Hardnefs,
lifiidfiefs, Heat^ and CoU j there is no Difficulty inaBy and Smooth.
other which may come under the fame Senfe, fuch as ”*

V.ongh and Fqfijhed. For all thefe Qualities do fo clearly ™

follow from the Difpoíition of the Parts of Matter only,
tiiit there is no need of any Explication of chem ; where-
fore 1 íhall pafs on to enquire into the Nature of Tajles.

CHAP. XXIV.

Of TJ s f E S.

^HE Word Tafle iXufed in Two Senfes. For Firft, t.TheMem.

it íignifies that Senfation which we commonly have

í’ -

wren we drink or eat. Secondly, we underftand by this
Word fomething, I know not what, in the Meat and
Drink in t^ch the Power ofrailing this Senfation of Taíle
in usj conmls.

2. Though Tajle in the former Senfe of the Word

2 . Tiaí aU

cannot be exadtly defcribed, ñor particularly known but

by Experience, yet y.^may make this Obfervation, that jameTafem

all Men have not the" fame Tajle when they eat the fame f‘‘”V

Meat; as appears from henee, that lome Men can eat

tvith Pleafure thofe Things which others have an Aver-

lion to : Whence we may conclude, that it is the fame

with Tajiing as with Feeling : For if we touch in the fame

Part, two Perlbns, the one in perfeít Health, the other

¡ufl: recovered of a Diflemper, they will be very difie-

rently affedfed, viz,. the one with an agreeable Tickling,

md the other with an intolerable Pain j in like manner

the fame Meat may caufe diíFerent Senfations in diíFerent

Perlbns.

3. As to Tajle in the other Senfe of the Word, which 3. AriflotieV
ive are principally to infift upon, Arijlotle’s Opinión is, %’"'»« «¡a-
That it is a ^ality or Propertji of a moijl Body arifng
'rom an earthy Drynefs, and a Heat on heing frejh hoiled,

This Definition contains Three.Things, every one of which
bave fome Refemblance of Truth. And firft, I think
iriflotle had Reafon to fay, that Tajle is a Property of a
noiíl: or liquid Body, becaufe thofe that are perfeéily dry
or hard, have no Talle ’till they are mixed with our
Spittle. Further, if we coníder that Water has fcarce a-

ny

syo--

RO H AU LT’s System

Part I.

4 : That Arl-
SuyÚQhas not
cxplained
abat Tajit is*

ny Tafc and Air none at al!, though they bff both inoitl:
Bodies, we rauíl confefs, that he had Reafon to add
fomething more grofs, and of an earthy Nature. , Laft.
ly, he oughc to bring in Heat, becauíe vve find by Ex-
perience, that in many Fruits it caufes oertain Taftes
which we did not perceive in them before they were
prepared.

4 . The Followers of Ariftotk will readily agree to that
Explication which I have given of his Definition of Tafif
but it muft be owned, that though he has faid nothing
but what is true, yet has he given us no Inlbrmation at
all j becaufe he has not explained what that AíFedionor
Property of Body is which caufes Tañe, ñor wherein it
coníiñs.

5- Some have attempted to fupply this Defedí, by fay.
Commenta- ing, that it is a Quality very like that Senfation which it
¿ttour” aware what Inconve-

nience this brings us into; For bebdes that this giveAo
inanimate Bodies a Mode of Exiftence, whifch does b*¿o
Means belongto them; it would follow from this Opinión,
that two Men could never have difFerent Tañes of the
fame Meat or of the fame Drink, contrary tÓ’ what we
have before proved.

6. On the contrary, íince we are already affured, that
when the fame Meat caufes difF^ent Senfadons in two
diñérent Perfons, one of them muft necelfarily have a
Senfation différent from' that in the Thing which raifes
the Senfation, we have Reafon to think the íame of the
other likewife. It is probable therefore, that the Facuiij
of Taft'mg in us, is very like the Faculty of'feeling Paiu ;
that is to fay, in order to bring this Power into Aói:, no-
thing more is required on the Part of thofe Bodies which
caufeTafte, but that they move ' the fmall Fibres of the
Nerves of the Tongue in fuch a manner as they oughc
to be moved, and as Nature has appointed, in order to
the Perception of Tañe ; the fame as in order to feel
Pain, nothing more is requifite but to move in a certain
manner the Nerves which are the Inñruments of Feelmg :
And becaufe noching can move another, unieís it be in
Motion it felf, and nothing can be applied to the Nerves
of che Tongue, fo as to have any Eftedt upon them, uu-
lefs it be of a certain Bignefs, and of a certain Figure ;

6. ThatTafie
con^Jls in the
'Crojnefs^ FU
^are and
Motion of the
Fítrts <f the
tohich
tajie.

K 7he fvail Fibres) Concernmg j Fart II. Chnp. iv. and the fétmm
the Organ ot Tañe, and irs Dcícrip. 1 Lewenhoük’s Epiji»
tÍGU. Se Re¿is’s Fhyf, ’Book VIIL j

Chap. 24 - o/ N atural Philosophy.

I therefore think, that the Form of a Body which caufes
Talle, confifts in the i Bignefs, Figure and Motion of its
Tarticks, and thac from :he DiflFerence which there
may be in thefe Three Things, there may arife different

Taftes. , •

7. And this is confirmed by a Truth, which follows
from what I have íuppofed, namely:, thac ifthe Partióles
ofa Body be fo fabril, that they will fcarcely or not at,
all move the Organ of Tañe, thac Body will have no
Tañe. And chas wefind by Experience, thac Water has
fcarce any Tañe, and Air none at all.

8, We may alfo give a particular Reafon why Air has
no Tañe, 'uiz. becaufe it fwims apon our Spittle without
mixing with ir, fo as to make any Impreffion apon the
Nerves of the Tonguej by which we may alfo under-
ftand why fac Liquors havc not fo íharp a Tañe as chin
Lifluors have.

171

7. fomt
'Bodicí Jtave
7 io Tajie»

8 . A parii^
cular Reafon
tobf ^ir hM
no Tajie.

ifli.

Furcher, if a Body be offuch a Nature,as thatñone 9- Whyhard

- — - - - • . . 4 . , "Bodiesfor the

mofi parí
have noTaJItm

ofThofe Parts are feparated from it, which are capable

of penetrating the Pores of che Tongue, inorderto move
the Fibre;t||f the Nerves, that Body ought to have no
Tañe. And fo vve find, in moñ Metals, and alfo in Glals
and Flint Stones.

10. Ñor are wetothink that there is any Thingin thefe
Bodies, thac caufes th'f%i to have no Tañe, but only, the
.not being divided j for the Sales which belong to the
Compofition of Glafs, tañed very ñrongly before they were
concreted ; and Metals which are reduced to a very
fine Powder by che Chymiñs, are of fo ftrong a Tañe as
not to be born.

11. Since Heac always increafes the Motion ofa Body^
and lince it is alfo very certain,. that che more a Body
is in Motion, the more capable it is of moving others
to which it is applied; it follows, that when Meat is
hot, it muft neceíTarily have a ftronger Tañe, than when
it is coid ; as every Day’s E.xperience fhows us.

12. It is alfo very eafy to fee, that the Heat, in ma-
king Meat ready, caufes the Particles to ñrike one againñ
another, fo that the Corners of many of them muft be

I O fíbtt; Me^
tais may ac-
atare a very
Jiron^ Tajie%

II. Why
xoarm Meats
have a firong»
tr Tajie than
thofethat are
coid»

I. The Bignefs, Figure and M.otion)
Oihers concend* that not all the Par-
ticles, but che Sales mixed with che
Particles of all Eodies, are che Caufes
ofallTaíles; which is handled ac
large by Mr. Le Clerc in his Phf,
"Boek V, Chap. xu. And indeed chis
ií a very probable Opinión 5 ’

whether the Particles of the Salt ón-
ly, or any other Particles, be the
Caufe of Tafles, it comes to ihe
fameThingj for werauílnecefláriíy
ac laíl: have recouríe to the Bignefs,
Motion and Figure of tho/e Parti-
.^ks. See tbe Notes on Art, 38.

12. Why
Me.at^ roben
it is made '
readyy has a
diferent
Tajie from
vohat it had
roben rato»

broken

t 72

TJ, Tffdt

thtre oiight
t^he a great
many tery
éiijferens
Tafes.

x^,yA74if-
takc of thoje
soh9 think
that all
*r¿3j2es artfe
Jrom a Mix~
tnre ef ttvo
JSxtremes^

T5'. TBdf
Svoeet OMght
not to be
fofed ta SíV-
tiv^

t 6 . What
Jlcidnefs con^
Jífista^

17. Why all
JFrnits he f ore
itiey are ripe
are fottre»

ROHAULf^s System Partí

broken off;. and they by this Means divided ínto fmaller
Particles tban they were befóte, and alfo of a difFerent
Figure; and this is the Reafon, why Meat, when it is
jnade ready, has a diflferent Taíle from what it had when
raw.

1 5. As to the DifFerence that there is in Talles ; finca
we have made thetn to confiíl chiefly in the DifFerence
of the Figures of the Bodies fo tafting ; of which Fi-
gures there may be infinite Variety ; this agrees with
Experience, which difcovers to us new Tañes every
Day.

■ 14. This being fo, I cannot approve of their Opinión,
who contend for two extreme Tañes, from a Compofi.
tion of which they imagine all others to ariíe. Belides,
that it would follow from thence, that all Tañes would
dififer oníy in degree ; which is contrary to Experience,
which flrows us, that there is a greater DifFerence
than.fo. I

1^. I do not fay that there can be no Inftances giveihf
fuch extreme Tañes, which raife in us the moft diñe-
rent Senfations ; but if any fuch afe to be allowed, 1
íhould rather opppofe a J^ar^ or ^««1 Tañciafip a éi/ier
Tañe, and not zS-meet to a Bitter, as is commonly done;
becaufe we do not find that a Jliarp Tañe arifes from the
Mixture of S'weet and Bitter-, but pp the contrary, Sweet
íéems rather to arife from the Mixture of the other Two,
as we experience in Fruits, the S'weetnefs of which feems
to be a Mediunjj betwixt an Jcid and a Bitter.

16. To attempt to treat of every particular Tañe,
would be to undertake a Thihg irapollible, and there are
many Things wanting in order to fpeak with Certainty of the
principal and moñcommon ones. Howeveramongft thefe,
íbme feem more eafy to be underftood than othersj fuch
zs Ácid or Soure like Citron-yuice. For as this Tañe pricks
theTongue, we may from thence conclude, that Bodies
which afFedt us in that manner, confiñ of a great Num-
ber of iong and Jiiff Bartules, which in fome meafure refem-
hle Jmail Need/es.

1 7. This will feem the more probable, if we confider,
that this fiare Tañe iscommon to all Fruits before they
are ripe; for this is a Sign that Sourenefs confiñs in fome-
thing which is common to them all; but we cannot con-
ceive any Thing elfe common to them all, but this
Difpofition of their Parts, for they are all compofed of
the Juice of the Earth, which ftops in the long ñreight
Pores of the Stock and Branches which bear the Fruits.

i8. That

Chap. 24 * Natural Philosophy.

i 8 . Thatwe may underftand fometbing of otherTaíles,
we may coníider the Progrefs of Fruits ’cill they come
to Maturityi for ifwe can but onceknow whac Figure
the Particles are ofj when we experience a certain Tafte,
it will be eafy to concludcj tbat this Tafte coníifts in
; this Sort of Figure. Firíl then^ fince all Fruits are ripen-
i ed by the Heat of the Earth and Air : (whethcr this HeaC
I becaufed by the Raysof the Sun, as coirimonly happens
' in Fruits that grow in Gardens, or whether it be produ-
ced by Fires kindled under or upon the Earth, as whea
Fruits are made to grow inHoufes, in the midft of Win-
ter ■,) we cannot help thinking, that a great many Partí-
; des of thefe Fruits are put into fo great Motion, as to
ftrike againfteach other in diíFerent Manners, fothat fome
of the longefl: of them are broken into íhort ones, others
have their Points only beaten ofE and others are made
entirely round. And then it is, that the Fruits have a
acid Tañe. Whence it is reafonable to conclude,
that the fcueet acid Tajie of Fruits conffls in this-, that fome
of their Particles are long and ftiff, and prick the Tongue,
at the foi'H^ time that a great many other of them are leji
fenetrating, and fo flip over the Fibres of the Nerves,
•éthout producing any thingmorethan a kindofTickling.

ly. We may obferve further, that the riper Fruits
grow, the more theilíl^articles are broken, blunted and
made fmall i wherefore fince the Fruits are then fweet-
er, we ought to conclude, that the great Sweetnefs of
Fruits arifes from henee, that they have a far greater Num-
ber of thofe Particles which can only tickle, than of thofe
which prick.

20. But if Fruit continúes ripening too long ; there is
no doubt, but that all its Particles will be fo bruifed, that
none of them will be able to prick the Tongue agreeably,
but they will only tickle it in a difagreeable manner :
Now Fruits when they are too ripe, become bitter ; whence
it is reafonable to prefume, that Bitternefs conffts in
this, that all the Particles are fo broken, bhmted, and madp
fmall to that Degree, that there remains no long and fiiffooies
amongfl them.

21. And this is confirmed from henee, that in thofe
Things which are made ready by Art, the Parts of them
which are burnt, and whofe Particles are beaten one againft
another, and have their Corners broken ofE are always
hitter, as we experience in Crufls of Bread, and in 'Roafl^
v/eat <¡^hen laid too near the Fire.

173

18. Whattbs
fweet acid
Tafle of
Fruit scotifjlí
in»

15. íiototísty
htcometnÚTt-
fyfweeta.

20. U 7 >at
‘Bitternefs
conjíjis iru

ir. Wby
Meats xshen
they are over^
made red^y^
betome bitter.

174

RO H AULT’s System

Partí,

2z. Why 22. The NaCure of Soure, S'weet and Biiíer

thus explaiaed, weihall no longer be furprized, thatfweeí
veiinto tmo ThingSj fuch as WTne, luppofcj, may be relblyed into
two other, the one of which is foure, or acid, the othet
ZhérUtter. '■> for cbat which makes any Thing Iweet, (with
fuch a Sweemefs as is agreeable to che Tañe) is compo.
fed of two Sorts of Particles, in che 07 ie of which Acidrieft
confiñs, and in the ocher, Bitteniefs.
í^. W^yíit- 23. Neither ñtall we any longer be furprized, that
Orange-Peal, Treacle, and many purging Medicines have a
atid&ngs heating Quality, and chat í7«íf Things, fuch as the Juiccd^

■ imiing. and Péryawj are commonly cooling i fincewearc

aíTured, thac Heat confiñs in fuch a Sort of Motion, as
the fubtih round and blunt Particles of hitter Things are
capable of exciting and continuing j and thac on the con-
trary, che long Particles, of which acid Things are cora-
pofedj being fomething of the Nature of Water, are more
proper to hinder Motion, thac is, to quench Fire, ian
,to kindle ic ,• wherefore they ought to be reckoneÍ*a-
mongñ coid Things.

24 . Hoto a 24. Neither is ic inconfiñent with what h^ been faid,
hitter Thing thac we fotnetimes find our felves cooler ú^á we were
before, upon eaiing bitter Things ; for there are fome of
thetn fo eaíy to be corrupted, that they can produce but
a very fmall Heat, fuch as is fc^ce to be perceived;
but yec this Fleac may be enougn to caufe fuch an Agi-
tation in the Particles of our Blood, as to carry oñ fome
noxious Matter which made it move too quick before,
and by this Means ic will be put into a more quiet State;
and thus we may feel the Heat abated, and our felves cool-
er chati we were before.

25-. Thntthe i-5- I ñiall noc infiñ any longer upon the Explícation
^heriitioncf oí particular Tañes. Ic would be very tedious to go
fSnth'M- t'’'rough them all, and require a great Number of very
ttratim of exad; Experiments, which I have not made, ñor perhaps
‘the ^pf-tí-f Opinión as much as 1

can, that their Differe?¡ce confifls in the différent Figum
of the Particles of the Body which we tajle ■, I will exí'
mine one particularly, and make it appear, thac as often
as our Reafon Ihewsus, that there is any Alteración in rite
Figure of the Particles, Experience íliews us alfo that there
7 . 6 . Tu- is fome Alteración in che Tañe,
frvw "‘and Example, and confider it

ehnr theiuod vcry Bcginning, ’till it degenerates into fome-

cj the riñe thiiig tliac is not at all like Wine. I obferve in the fitll
cnrnt n-t te jj-jg Mo'fuTe o¿pche Eatth, bccaufe it is com-

tnay he cool-
iw.

of the TioHy
•tohich we
Kaf.:*

haz-e any

175

Chap. 24> o/NaturalPhilosophy,

pofed of the mofl: minute Particles of ir, has fcarce any
Tafte, and though in the Pores of the Wood of the
Vine it grows in groílér Particles, and fuch as are able to
move the Nerves of the Tongue j yet becauíé it fticks
among the Parts of che Wood, and is not eafily diíinga- .
aed from it j therefore it excites but a very fraall Sen-
fation in thofe who chew the Wood.

L 27, Further, fince the Particles of the Juice which 27- a

I get into che Air and diíiill through the Stalk of the craptXien
Bunch, in order to form che Grapes, ftick together, and « i, firji
cannot eafily be feparated ; it follows, that they can ap-
ply themfelves to the Superficies of the Tongue oniy, lUticrlfi/.
and confequently that they can raife but a fmall Senlá-
tion fcarce to be perceived. And ib \ve find by Expe-

nence. ^

28. But fome time after, when the Particles, of which 28. mmre
tlie fraall Grapes are compofed, are feparated from each

ofter, either by the Heat of the Air which agitares them ofFerjíict. *
glrtly, or by the Acceílion of more íimilar Particles which
tliruft themfelves in to increafe the Bulk of them ; it is
inanifeft,^at they oughc then to añ feparately, atid to
raife the Snlátion of a very íliarp Tafte, fuch as we ex-
perience in Verjuice.

29. And the Heat of the Air, which increafes as the 29. ííij»
Fruir ripens, contiv^ng to move the Particles of the

Grapes, it is evident, that they mufl; be more and more ^ ‘ '
blunted thereby, and fome of them made fo very fmall,
as only to tickle the Tongue agreeably, and to excite that
Senfation of Swcetnefs which we feel in chewing the ripe
Grapes.

30. We fee alfo, and it is an Obfervation worth taking 30. P'vkatcc
Notice of, that if it be wet Weather about the Time of
gathering the Grapes, the Water which finks into theifit rains
Eardr, will afFord too much Nourilhment to the Grapes ; ring the Vin-
Wherefbre as there are too great a Number of long Par-

cicles, which there is not time for brealting or blunting,
it follows, that the Grapes will not be fo fweet as they
would otherwife have been. And chis is often found by
Experience ; For if it rains a little beforethe Vintage, the
Wine is fharper, or,' as they cali it, harfiier. This the
People of Languedoc feem to be aware of, who are at the
Trouble, a little beforethe Seaíon of gathering the vtufca-
iine Grapes, to twiñ che Stalks of all the Bunches, that fo
they may ripen, and not rcceive any more new Nou-
riílrment.

31. For

'i7& ROtl AULf' s Systim Párti,

■^i. The Rea. 31. For a further Confirmation of what I have faid

**• worth obferving, that if we taíle of the Juice of tht
fweet. Grapes jufl after they are prejfed, thece oughc to be ver»
little DiíFerence from the Talteof che Grapes themfelves'
and ít ought alfo to continué its Sweetnefs for fotne time
after ic is put into che VeíTel, provided the Veííel be well
ftopped. For though^ while ic is working, many of che
long Particles which are intangled in one another,. havci
an Opporcunity of getting clear^ and fo are capable oí
pricking ^ yec however they cannot caufe any íharp Sen-
fation^ becaufe they a¿t in Company wich a great many
ochers which have had fufihcient time to be broken and
made fmalh having been preferved in the VeíTel care-
fully ftopped up : And this agrees very well with the
fweet Tafte which wc find in Ne-w ÍVine befare it ¡i
jined. *

í2. Why 32. If while the Wine is worklng in the Prefs ot
mnegrows ajjjj while ic continued to work in che Veíreh||ie
mZkia¿Z fabril Particles, which h’ave moft Motion, and wlich

by reafon of cheir Smallnefs were lels ingaged wich the
other, be permicted to fly away, and evaporare into the
Air through the Bung-hole, which is left o^ib for chat
Purpofe, there muftneceíTarily remainfewerofthofe Par-
ticles which tickle the Tongue, and more of thofe which
prick it. And this is the Reafon ^?hy we oughc chen to
find che Tañe fharper, tbac is, fuch as we experiencein
ff^»e nat quite fit to drink.

33. Hmh After chis, weniay confider the Wine in two Con-
YZméjí. ditions : Firft, let us fappofe it ftopped up in the Veflel,
fo clofe, that it has notthe leaftCommunication with the
excernal Air ; in which Cafe fome of its Particles will be
broken and blunted, and a great many of thofe which if-
main whole, willlofe theirStifthefs, and become plyaole,
^ by rubbing againft one another, and bending in that ñrait
Place in which they are inclofed ; and by this Means they
will be lefs capable of íhaking theNerves of the Tongue;
Wherefore the Wine will no longer tafte Sharp, ■ but at-
rain that Sweetnefs which we experience in ic when it is
fit to drink.

, 4 . Hew it ' 34 - withoutdoubt the Sweetnefs would increafe
becams concinually, if the Woodof the VeíTel did not change the
i/tryf«>¡ct. Liquor a little, and permic the more fabril Parts of ic to
evapórate through its Pores. For a Proof of which, we
may remember, that Wine kept many Yea'rs in earthen
Bóteles, well ftopped, and put into Sand in che Bqttom of

Chap. 24 * o/ Natural PHiLOSOPHy,

the Cellar, wEl in length of Time become. as fweet as
Honey.

35. Suppofe nowj that the VeíTel be not ftopped j the
long Partióles which íllp by one another, may be fo worn
as to be a little diminiflied, but there is no Neceffity that
they íhould become limber and pliable : For thofe of
them that are moíl limber, are at liberty to evapórate
through theHole of the VeíTel, and thofe which remain
llave the more room to move in without being forced
to bend thcmfelves. So that all the Alteraticn that will
happen to the long Partióles which remain, is, that they
will becorhe more Iharp, and the Wine will be convert-
ed into a Liquor which will prick'the Tongue more iharp-
ly, that is, it will be turned into Vinegar.

35. If the Particks ílill continué to be thus moved for
a coníiderable time, they will at laft be fo worn, and be-
come fo very ílender, as to be extremely pliable, info-
fiuch, that they will have no Power at all to move the
Hierves of the Tongue , and then the Liquor compoíed
of them can have no Talle, and be very little diíFerent
from Ví^er ; as we find by Experience.,

37. a final Confirmation of what I have faid con-
cerning Taíles, I will relate an Experiment which I made
my felf: I took a Pewter Pot, and having made a
Hole in the Botton^of it, I ftopped it with a Piece of
Cloth, and then mfed it abouc half full of very fine
Sand, fo well wafhed, as not in the leaft to tinólure the
Water which drain’d through, and afterwards well dried ;
After this, I put in a Quart of full-bodied Red-Wine,
which diftilling through the Hole below, there carne oút
about a Pint of clear Liquor like Water, which, had no
Tafte: Then perceiving that tha Drops beganto be tin-
gad with Red, I took away the Veflel which I hadfet un-
der, and put another in its Room, into which there ran
pretty near the other Pint ; and this laft was much leís
red, and had a much fainter Tafte than the V/ine it felf
before it paíTed through the Sand. Laftly, mixing thi 9
Liquor with the other, which w'as very clear, the Refulc
was a Liquor of a very faint Colour, and fcarce any
tafte.

38. I think no Body that knotvs what Sand is, can find
out any other Reafon for the Alteration of the Tafte of
the Wine by paffing through it but this, that the Parti-
cles of the Wine being forced to go through very nar-
row winding Paííages, are bent a great many times all

N ^ Ways^

177

grow

fhar^.

55 . Hoto Vi-
negar may bg
turned into
a Liquor that
may havg no
Tafte,

37 . ^ re*
viarkahleEN-
perimtnt* _

38 ^ The Con- -
clufon oj ibis

Chapteu

'378

ROHAULru System Part!.

Waysj and ' have the Figure and Condición of thetn
changed : From whence we may conclude, thac ^ the Form
of all Bodies that have any Tajie tonpfls m the Difoftm
and Figure of their Farticles.

I. Have the Figiixfit &c. TheFi-
gure of themis noc alteredj butonly
ihe Parts which have no Colour or
Tafté, are feparatcd from che red
•Parts which have a Tafte.

2 .. The Form of all 'Bodies^, &e.}
Thac Tafte confifts whollyin the Fi-
gure and Compoíitlon of che Parts is
clearly demonftraced by che famous
Mr. Boyle, from che lurprizin^ Al-
teration of Taftes, by variouüy
compounding of Bodies. I chink
it worrh while briefly co propofe
rhe Experimencs made by thac ex-
eellenc Períon, becaufe chey oughc
to be kepc in Memory.

Firft: From tm BodieSi one of
which ts vcry acid and corrojivej the
other alkalioHS and fie.ry y may arife a
Bod^ wííhofit almo/i any Tafie. This
ís done by a cercaín Compoíition of
S^irit of Nitre and Nitre fxed per
deliquium.

SecoDcIly. A Body that has fcarce
any TaJle may he feparated into two
Bodies of a very Jhary Tajle^ yet
very digercnt from each other, This
is done by diñilling the moft refi-
Ded Salt of Hitre by Tnfiammationj :
or wich a Mixture of'Clay which has
itíelfno Tafte.

Thirdly, From ttoo Bodiesj ene of
tohich is very bittcr, and the other
very falt, may arife a Body which
hasno'^fe, This is done byíprink-
iing Cryfials of Silver difíblved in
Aqua-fortis with Brine or ,Salt
Water, and then melting and prepa-
ring them on the Fire till chey come
to a Lunea Cornea as the Chymifts
cali ir.

Fourtbly, From tm Bodies mixed
to¿ethery one of which is very fweet,
and the other very falt, may arife
alfo a Body which has no Tafie. This
;is done by pguring a certain
«Quanticy of Spir¡8 of Sal Ammoni*

ae or Salt of Vrtne upon red LtaA
difíblved in. Vinegar, or Sugar j/
Lead difíblved in a proper Mvu
Jlranm,

Fifthly? From two Bodies, one of
which is acid, and the other has «o
Tafle, may arife a Body very Inter,
This is done by ftraining Aquafor-
tis íáturated wíth difíblved Silv&r\
For ic will afford very Meter Cry
fiáis»

Síxchly, From two Bodies mixti
together, one of which is infipid, md
the other very corrofive, may arfe a
Body fweeter than Sugar, This ií
done by pouring the beft Aqua-for-
tis upon red Lead, and then pm-
ting ic over a moderace Fire cA
is facurated. ($/

Seventhlyj From the fweetífi Be.
dies ofall, without mixing any othn
Bodies with them, may,-he extraSei
very corrofive LiqtiOT$,i-f}ich as mili
difiolve certain Bodies, Thus a Spi-
ric chac will diflblve Copper may be
excradled from or Honey»

Eightblya A Body as bitter asían
he, majCfi feparated into two Bodies,
one .of which is very acid, and the
other without any Tafie, Thus a ve*
ry acid Spirit may be extradled from
Cryfials of Silver diftilled over a very
hot Fire> and a Body without ány Tafe
will remain ac che Boctom.

Laftlyj The fame Body dijfolved ¡n
dijferent Liqmrs ,as Aqua-fortis, Acua*
regía, Spirit of Salt^ difiilled Vmu
gax, Spirit of Uriñe, &c. will have
a diferent Tafie in each of them. So
alio, the fame Liquor, as Aqua-fortis,
mixed with difierent Bodies, will have
different Tafics, thus with Silver it
will be bitttri wich Lead it will be
fweet, wich Copper ic will be hito-
lerable» See Boyle of the Frodttiii^'it
of Tafies*

C H A P,

I

Chap. 25* c/ N ATURAL PhILOSOPHY,'

C H A P. XXV.

0/ S M E L L S.

B Y the .Word Smell, we may firffc underlland thac par- >•
ticular Sort of Seníation which is raifed in us by the
Impreffion of ceirtain Bodies upoti t the Nerves of the in-
ternal Parts of the Nofe: And we may alió underñand by
it, that in the Body which fmells, in which the Power of
exciting the Senfation of Smell in us, coníiñs.

2. Every Body knows by their own Experience what
Smellis in the former Senfe of the Word, but it is im- smeii'hmt
poffible to defcribe and make fuch Preception known to aiike /» ail

f thers. All that we can fay, is, that the lame Object does
pt raife the fame Senfation in all Perfops, a great many
nding certain Perfumes agreeable to them, which others
cannot bear.

3- being fo, we (hall only endea vour tofind out
what S 7 neU is with refpeft to the Body fmelling. Ariflotk dcfined what
has not defined it at all in that Chapter where he treats Smeiih,
expreííly of Smells, and ^ where he makes this Excufe,
that Men havenot hílirSmell fo perfeót asotherCreatures.

4- Some of his Followers think they underftand what
he means 3 from that Place where he íays, Ihat the In-
fiant we perceive any Thnig, we become like the 0¿-,
jeíl which aBs upen us to caufe that Senfation : And
upon this Foundation it is, that they contend that Smell
in the Objedl is fomething very like that Senfation which
itraifes in us. To which theyadd, that arifes from
the Mixture of hot and coid, dry and moiít, but fo that
the hot and the dry prevail moft.

5- But befides, that this Opinión afcribes to inanimate f, a Confa-
Bodies, a manner of Exiftence which agrees to thoíé
only that are animated, which cannot bej it would fol-
low, that the fame Smells muft be equally agreeable to
all Perfons, contrary to what was obferved before. To
which we may add, that it is wholly inconceivable, (fup-

1 . The Nerves of the internal Farts")
For che Organ of Smelling, aad the
Defcripcion of íc. See Regis Fhyf.
3 S. 8. PdrfU. C/jíip. V.

2. Where he makes this Excafe)
It is not fo evident xohat Smell ts,

Darknefs or Lip^

Coloar are. The Reafon ist hecaufc
we have not this Senfation very per^
feéii bift voorfe than many other Sini-
mals i For Man^s Smell is very badf
Arift. de Anima, lib. a. cap. 9.

g, Fromthat Fíate) See che Note#
on Chap, An. 7.

pofing

xSó RO H JULT’ s System Partí,

pofing the Idea’s which the Arifiotelians give us of the
íbur principal Qualicies that come under the Senfe of
Touching to be true) that the Mixture of them lliould
produce any Thing elfe but Warmnelsj which will be
more or lefs dry or moiftj accordíng as it has more or
lels of thofe Qu'alities mixed with it, which has no Simi-
fitude at all to that Idea which they give us of Smell.
LafUy, If this Mixture were Smell, as we perpeive it by
. Touch, it ought to raife a Senfation liíce to it felf in all
■ Places where the Organ of Touch is; and then we ought
to fmell with our Hands as well as with our Nofes 3 which
is contrary to Experience.

What the 6. If to this it be anfwered 3 that That which caufes
^meluclL Senfation of Warmnefs, when it adfs upon theHand,
fij}s in. ' may alfo excite the Senfation of Smell, when it adts up-
on the Nofe, Nature having fo ordered it ; I agree with
them. But becaufe I know nothing elfe in Bodies bujt
Magnitude, Vigur^ and Motion, I cannot think there is ne^
of fuppofing any Thing elfe to make them capable of ,
impreiíing Smell upon the Organ of SmelHvg: Wherefore
I am of Opinión, that the fame Partióles wjrict^.ife the
Senfation of Talle, when applied to the Tongue, may
alio raife the Senfation of Smell, when being fo very
fmall as to fly about like Vapours or Exhalations, they
come to tickle thofe two extendedP'Parts of the Brain
which anfwer to the rabil; inward Recefs of the Nofe.

7. my 7. This may be proved from henee : Firil, That we
^eihare^ , experíence, that the greater the Heat is, and confequently
v7dlíln'ft fhe more capable of making a greater Number of fuch
is hot, thaii Partióles as cauié Smell, to fly ofF 3 the further do Bo-
extend their Smell ; And on the contrary, as the
Coid keeps their Partióles at reíl, and hinders them from
exhaling, fo it is^e'Caafe of their Smell’s being leis per-
ceived. .

z.iVhyuT- 8. Further 3 weobíérvif, that a great many Bodies fmell
than whilft they are moift, th^t is, fo long as
ceajefojme., Motioii 3 and that they

ceafe to fmell when they are quite dry, or have all their
Partióles at reft.

5. i-hafio- p, Laftly, One of the moíl evident Proofs thatwe have
féem”'to“Lve Smells coníiñ in the EvaporatiOn of certain

710 Smell, may Particlcs, isrhis3 that moft hai'd Bodies, which do not of
^(me ílneU 2s we fay, raife the Senfation of Smell, when

^ ' they come to be burned, or only to be rubbed one a-

gainíl another, appear to have a Smell 3 becaufe by thefe
Means forae of their

•Chap. 25* Natural Philosophy. í8i:

Thus Sealing-Wax, when it is lighted, raifes a Smell,
which was noc perceived before. And chus Iron rub-
bed againft Iron, and one Flint againít anocher, raife a
Smell alfo which was noc perceived before.

10. I do noc however precend co affirm, chac all Sores 'o-

I of Parcicles which are carried ofF from all Sorts of Bo-

dies, oughc indifFerencly co raife Che Senfacion of Smell ¿ any SmeU.
For in order chereCo, there oughc co be a certain Mocion
of che Organ of Smelling, and a cercain Forcé co fliake ic
and chere may be alio Parcicles fo very fmall as noc co
be able co íliake ic che leaft chac is poffible; Thus, che
Air which we breach, arid che Vapours-which rife ouc of
Wacer, have no Smell ac allj and, on che concrary, chere
may be ochers ío large as chac chey may noc come co che
Organ ac all, or if chey do come co ic, are racher capa-
ble of quice fpoiling ic, chan of Ihaking ic in fuch a man-
ner as may raiíé che Seníacion of Smell.

® II. The Difference of depends upon che íáme
^aufe as che Difference of Talles does, chac is, i upon
the Difference there ts tn the Eignejs ana Figure of the Par- jifis.

_ N 3 ticles

1 . Üpm the Difference) That
fmell, in che íame manneras Tartesa
coníiíts entirely in the Compoficlon
and Figdre of che Paresf/^ very evi-
deot from che following Experiments
made by che Famous Mr. 3ny/¿.

Firftj From txoo Hodies mixt
together, each of which is volth-
oiit nny Smell, may be raifed a very
frorig arinofís Smell, This will be, if
mjlacUed Lime and Sal Jlmmoniac
bebeacen togeiher. ^

Secondly, "By a Mixture ofcommo7¡
Water, which has no Smrll, a Boáy
which has alfo no Smells may be made
4o fend forth prefently a Jirong Smell.
Thus> Camphire diflbived in OH of
Vitriol has no Smell. buc mixed» wich
Water, it immediately fends forch a
ftrong Smeil.

Thirdly, Compound Bodlcs may fend
forth Smells which are not at all hke
the Smells rf the Bodtes feparate.
Thus OH of Tnrpentine mixed with
double the Quanticy of OH of Vitrirl,
after icis diftilied, will not fmell of
Turpentine buc of Brimftonei and
chac which remains in chg Retorcj
ific be forced wlth a ílronger Fire»
will refemble che SmeU of diíiiUeJ
Oilof Wax.

Fourchlyj ^ great many Smells
VJay be raifed only by Motion and

' N>

Zdgitation, Thus a Multitude of
Bodies, as Glafs, Stones, &c. whicha
chough heaced, íénd torch no SmeU,
yec if agicated and bruifed wich a
particular Mocion, fend forth a very
Itrong Smell 5 and chere comes a
Smell like that of a Hoíé, ouc pf
Beech‘toood while ic is turning,

Fifthiy, .A Bady that has a Jirong
Smellj mixed with anoíher Body that
has 710 Smell, may lofe all i:s own
Smell, Thus if A^uafortis not coo
weil dephlegmaced be poured upon
Sa/t of Tartar, cUI ic ceafes férment-
tng 5 thac Liquor, after evaporación,
will afford Cryflals W'ichouc any
Smell; like Sale of Nitre 3 buc if chey
De burnt, chey fmell as bad as can
be. ^

Sixthly, Ont of two Bodies mixed
together, one of which has the worji
of Smells, and the otker not a very
good one, may arlfe a pleafatit aro~
maticli Smell, This is done by a
certain Mixture of .At^na fortis or
Spirlt oj Hitre with imjiammahlq
Spirit of Wine.

Seventhly, Spirit of Wine mixod
with a Body that has fcarce atty
Smell, may prodtxe a pleafant aro-,
maiiclt Smell. Thus an e^ual Quan-
City of infiammahle Spirit of WHjé
and Oyl of Damsciik P^itriol, mi>:ed
togstheri

1>

sSá RO H AÜL7*'s Sy'stem Partí,

ticles •which are exhaled from the Body that fnielh. As will
be evident to any one Who confiders that thofe Things
whicb have the fame Tañe, have alfo the íame Smell •
Thus all fliarp Bodies have a flaarp Smell, and all bit-
ter Bodies have a Smell that has fomething of Bitternefs
in it.

íz.iTmtií 12. And this is fo true, that when we are once aíTured
fame'Boiy Particlc of certain Bodies have changed their

fitthd^erínt Figutes, wc always find by Experience, that they have
SmciUmeaf- changed their Smell alfo. Thus, the Matter gendred in
ur muher. Abfcefs of a Land Beaver, expofed for fome Days
together in the Sun, in a hot Country (which without
doubt dañies the Parts one againft another, and alters
their Figure) fenfibly alters its Smell, and as ftrong
as it was, it becomes firft tolerable, and at lañ is
turned into that valuable Perfume, which we cali'
Mush.

13. From what we have faid concerning the Nature
¡mdiiig 3o- fmelling Bodies, we may conclude, that both their Bulle
dies dimi- _ and their Weight diminifh by little and little. Thus we
’s'handUttk. ^7 Experience thofe Smells to be quickly ov^|Which
are raifed by burning; Butas to thofe which we^perceive’
without Jieating 'the Bodies, fuch as thofe of Musk and
Ci-vet, I it is a long time before they are fenfibly dimi-
nifhcd, becaufe the Motion of their j^yticles is very flow,
and but afewof them are exhaled ata 'time. And as but
a fev/ ofthem are exhaled at once, they couldnotmovethe
Senfe, vdthout meeting and mixing with a great many
others, which were fometime before evaporated, and flew
^ about the fmelling Body.

together,* and difíeíled, and then dx-
ílilled, wdl afford a penecrating
í>píritof a very pleafanc Smell.

Kighchly, 'Eody rfthe mojlplea-
fíint ■cohhoiit mixing any other

’Body voith it^ may degeneratc inio the
voorji Jiink» Thus ihe Spirit ■menú-
oned in the foregoiug Experiment.¡ if
ic be kept ítopped up in a Bocele,
■will ih a íhorc tíme degenerate meo
the ílrong Smell of Gariick.

Laftly, Oiit of tToo 'Bodies.^ one of
tobich has no Smell, the uther a bad
Smell, may arife a pleafant Smell
l'ke that of Mnsk. This is done by
putcing Pearls into Spirit of Vitriol'
while they are diíiblvir^j they

fend fbreh a pleaíant Smell. Sea
Biyle of the Erodndion of Smells,

I. It is along time.j 6cc.) "Wbo-
ever conGders che infinite Divifibili-
cy of Macter, and che inconceivable
Smallnefs of the Parts of Lighc which
always nnd an eafy and open Paíláge
chroiigh Glafs and Diamonds on all
Sides, and everyWay, will, I believe,
have no düubc, but thacic is whol-
ly owing to che Smallnefs only of
the Partides ’emitted, chough they
may be very much larger than the
Partides of Lighc, that Bodies-which
have a Smell, are yec a very long tíme
belore chgy are fenfibly diminiíh’d.

c H A P,

V

%

Chap. 26. Natural Philosophv. 183

C H A P, XXVL'

Of S O U N D.

T he Word SoukJ was intended to fígnify in the

firft Place, that particular Senfation which is raifed tZ”u 2 a-
in US, by the Impreffion made upon the Ears by what ir>¿s.
v/e cali founding Bodies. And the fame Word is alfo
ufed to íignify That in the founding Bodies, as in a Bell
or in the Air which furrounds it, which caufes in us the .

Senfation of Sound.

2- After what has been obferved when wé fpoke of
Talles and Smells, it is needlefs to fay, that Smind, taken heretZm^r-
¿p the former Senfe of the Word, cannot be defcribed,

^r known any other Way but by Experience. Where-
fore we fhall treat of it here only as That in the fotmding
Bodies or in the Air, which we cali Seund.

3. A 0 :otle has ' a Chapter particularly upon this Sub-
iedl, wherein he aíTerts, that Sound is jiothmg elfe hut the ¡mnl
local Motion of certain Bodies, and of the Médium applied to
the Ear j and that we may be fure that this is his No-
tion, he repeats it íl^ove twenty times.

4. I take particular Notice of that extraordinary Care ^ffheNotka
which Ariftotle took, to make us underíland the Notion FÍiiZenZ'*
he had of the Nature of Sound. For though he repeat-
ed it fo often, that it may feem troublefome to íbme Rea-
ders ; yet I find, he has not faid it often enough for fome^
others, who profeíling to follow his Opinions in other
Things, do notwithílanding believe that Sound is a Quality
diflFerent from local Motion

5. Thereare fome, who, to maintain this Opinión, and s-WhatRea-
confute that of Ariflotle-, fay, that if Sound be nothing elfe thygfvt
but local Motion, it would follow , that in moving our
Hand, for Inflance, we ought to perceive fome Sound ; and
there are others who aílért, that according to this Noti-
on, it muíl follow, that a Bell which is heard two Leagues
every Way, muíl move the Air fo far all round, which
they think abfurd.

6 . Flowever, thefe Objeítions are of no Weight ; for 6. That-thy
as to the firíl, it proves no more than this.3 that Sound ff^fcrinz

from Ariíie*

I. ^ Chapter particularly npon this j cerniDg theSi!»/’. jle,

Sahjeíl) Cbap. viü. Book 2. Con- |

N 4 does

1 S,4

RO H AU L'í’s System Pait I.

7 . That the
foundin^ 3o-
dy does fiot
cauft all that
Moúon xohich
is reqstijite to
produce
Soand,

8 , it is
not at all dif-
ficult to ptit
fome'Eodits in
JiAotioniVohich
Jeem . hArd to
iv moved.

does not coníifl: in all Sorts óf Motion, and efpecially not
in fucha Mo’rion asis given to tlie Hand when it is mo-
ved ,• which indeed is very tnie. And as to thofe who
think it abfurd, that a Bell íliould tnove the Air for two
Leagues round, they judge of Nature only by their own
Prejudices, which are no Proofs.

7. I confefs indeed, that fome Forcé is required to
put a Mafs of Matter, which is extended two Leagues
round in Motion,- But the Effect produced by the Bell is
not fo great as we may imagine ; For when it moves the
Air in this manner, i it only aóts upon a Body, which
was in Motion before as it is a liquid Body, So that
it does not lo much ad upon it to give it Motion, as
to determine that Motion which it had befóte, in fuch
a Manner as is proper to produce in us the Seníation of
Sound.

8. I fay further, that it is not fo difficult as is imagi-
ned, to caufe fuch Sort of Trembling in a Body whicji
is every way furrounded with a Liquid ; Experience íhovw
US this in a large Anvil, (which doubtlefs is one of thofe
Bodies which are not apt to be put in Motion; Vd>r w¿
fee it tiembles upon the leaft Blow given it by tné Ham-

mer ; and we mav obferve, that if a few Grains of Mil-

p. That
^onnd conjíjis
in a ctrtjm
fort of Moti-
m only^

Jet be put upon it, and ir be ftruck on the Side with a
modérate Key, according as the Sqj^d is more or lels,
the Grains of Mület will jump higfíer or lower, and
change their Place on the Anvil. Now it could not
caufe this Motion in the Grains, if it was not moved
it felf.

9. And to íliow that Soimd cónjifls m a particular Sart
of Motion, we need only coníider, that it is always pro-
duced when we ftrike our Fingers over the Stringsof a
Lute, or when we ftrike againft any hard Body. No\y
to ftrike the String of a Lute, or to ftrike any hard Bo-
dy, is nothing elle but to move the String out of its
Place, or to put the Body in Motion. And it is very
abfurd to think, as the Arijiotehans do, that the Conftitu-
tion of them is altered, and that we make them to ac-
quire fome Heat or Coid, fome Drynefs or Moifture which
they had not before.

r. ttonly .aüs apon a ’Bodyfj The
which is ín the Air before
it is moved by che Sounding Bod/j
•concributes nothing ac ail cowards
prodiicing Sound. For as many. Par-
cicles of Air as there are, tencling che
Umt v/sy as they are impelied by che

founding Body, and therefore more
eafily yielding to it than it they
were ar reíl i juíl fo caany oughs
we always co think are moved -che
contrary way ; and for that Keafon
reííft che Bodyin Motion more than
they would do if they were ac reíí.

lo. And

Cbap. 26, of Natural Philosophy.' 185.

10. And this is confirmed from heneen thac if the Ear 10. ^ Pmf
be tickled in the iníide fo as to make any Irapreffion up-

on what * the Phyíicians cali the auditory Nerves, we find
a cercain Tingling. Whence it is eviaent, that it is the
fame with the Senfation of Sound as with that of Pain
and both the one and the other íhowsj that by the Ap-
pointment of the Author of -Nature we are made fo,
that when certain Nerves are moved after a particular
Manner, we fhould have a particular Senfation.

11. I can’t otnit here an Experiment which is often if- -Arntíet
made ufe of to, diverc Children, and which wonderfully
confirms this Opinión. They püt a longThread through

a pair of Tongs, and wind each End of the Thread
about their Fore-fingers, and then ílop both Ears with
thofe Fingers j then moving their Bodies backward and
forward, they tofs the Tong? in the Air, and hit them
againll the Andirons, or any o’ther hard Body. Now
tlMgh thofe that . ftand by, hear but a modérate Sound,
yerthe other hear a Sound as loud as that of a large
Church-Bell. It is impoílíble to folve this any other way,
but by ¡ipng, that the Motion of the Tongs fliakes the
String, which gives its ImpreíEon to the Fingers, and thcfe
move the Parts of the Ear, to which they are applied, and
by this Means the Nerves of the Organ of the Ear are aF
fo moved.

12. Being aíTured that. Sound confiñsonly in fo?>ie Sort n- ^ Mjf.
é Motion, all that remains, is to determine what Sort

Motion that is ; And here i cannot agree with Arijutle ¡hs Sntjra
who woLild have Sound to be the Motion of a Body that »/
is hard, poliffd and cancave ; for it is certain, that there
are a great many founding Bodies which thefe Qualities
do not belong to ,• and alfo, that there are nene of them
in Gunpowder when it takes Fire in a Cannon, which
' yet makes fuch a prodigious Noife.

13. Some'perhaps, out of Zeal to this Philofopher, may 15. Tha
attempt to defend his Opinión, by faying, that if thofe
Qualities required byTiim in a founding Body, are not FouZ”Í. “
to be found in the kindled Powder, ñor in the Air -^ndofhn-
wliich is íhaken ; yet they are in the Cannon, upon
whichhe would make theWboleof the Sound to depend.

But without amufing one’s felf to find out Reafons
to confute this Opinión,- it iliall fuffice to alledge the Ex-
periment of what the Chymifts cali Aurum Bulminans.

What they cali fo, is only a Compufition of three Parts

Tht Phyjidans cali the auditory I Hearing and ics Deícriprion, See Re-~

Rtrvii) CüUcefning che Organ of I ¿is Phyf, Vlll. Part II. Chao. vij.

of

íS6';

T4, T^jat
Sfífíníi con-
Jijtiín a par-
iíCnUr forí
ef iWííww,

1 5-. TÍJJt this
^./of jü?5 may
bi conjídered
itt thc fonnd-
r>;" ‘Bfidj^and
TU í/;í Mfidi"
um.

26. ^hat ihe
^íitnd of the
String of a
lAtts conjlfís

RO H AU LT's System Partí,

of Sak-Petre, two of Flovrérs of Sulphun and one of
Sak of Tartar, beaten fepárately in a JViortar, and theii
mixed together. We muft take abone as much of this
Mixture as we do of Gun-powder to prime a Musket,
and lay it upon an Iron-PIate, or a fíat Tile, and put it
upon a Chafingdin-i of Fire ,• then che Powder will grow
hot gradually, and be ac once 1 turned into a Fíame,
which dilating it felf every way, caufes a Sound almoll
as loud as the Report of a Muskec well charged. In
this Experiment, the Iron Píate or the Tile, ferves only
to hinder the Powder from taking Fire, /cill it is equally
heated all oyer j and íince the Sound depends upon the
Fíame and the Air, which are neither hard, ñor poliflied,
ñor concaye, v/ichouc doubt this Opinión of Arifiotlii
cannot be fupported.

14. We choofe rather to fay, that Sound confifts in
a particular Sort of Motioh of Bodks, than to fay with A-
riftotk, that it confifhs in the Motion of. a particular Soihj
Bodies. For a more diñind Explicación whereof, wefflay
obferye, that the Bodies which we cali founding Bodies,
are noc applied immediately to our Ears, in ^"tler to ex-
cite the Senfation of Sound, but for the moñ part a£t by
the Interpofition of the Air which they put in Motiqn ;
wherefore we ought to find ouc what the Motions of
each of thefe are, when they prfji’aice this Senfation in

US.

15. There are Tome Inñances in which it is eaGer to
find out the Mariner in which the founding Body is moved;
and there are others in which it is eaíier to find ouc the
Motion of the Air. The former of thefe we will íirft
explain as far as we are able, vix,. the Manner in which
fomidmg Bodies are moved-

16. And to begin with the Lute, ór any fuch kind of
Inftrument that is plaid upon with the Fingers ; it is to'
be obferved, that the Stringsbeingftretched, are asñreight
as is poíTible, and that in playing upon chcm they are put
out of their Pofition, and benc a little by the Fingers ;
but as foon as they are let go, they return again to the
Place out of which they are moved, and the Velocity
which they acquire in returning, malees thcm go a little
beyond it ; then they come back,and go a little beyondthe
Place of Reft again ; and thus they go and come feveral
times, or ha ve leveral Vibrations, and in this trenilil¡!i¿
Motion conliñs their Sound.

T. Turned intoa Fiame) See thrs | en Tari IIL Ckap» ÍX* Art, 13.
Plirenomenon explained in :he i^'otes I

17. Thfl

Chap. 26. o/ N ATURAL Philosophy. . 1S7

The Sound of the Strings of a Violín coníifts in ^^•Whatthe
the AgiCation they are put into by che moving of the
Hair of the Bow over them which is made rough and wl/w ÁLf
jaggedj almoíl like a Saw, by being rubbed with Roíin.

Which is fo trucj that if the Hair of the Bow be rub-
bed with Tallow or Oil, the Strings will have no
Sound. becaufe they ílip under it, and are not íhaked by
it.

18. The Sound which a Drinking-Glafs makes when ^^-Whatthe
the Finger preffing hard upon it njoves round the upper

Edge of it, coníiñs ín the Vibrations like thofe of che G/afs”j^j
Strings of a Violin ^ it being evident. that the Finger here
fupplies the Place of a Bow.

19. The Sound of a Bell coníiñs in aTrembling, pretty 15. mat

much like that of che String of a Lute : For it is certain, »/

that che Blow given it by the Clapper alters its Figure a

little. fo that from being round, it becomes oval ; And'^'*
biíaufe át is tnade of Metal very ftiñ" and fpringy ,• that
Pa* which is moft diftant from the Center. returns to-
wai'ds ir, and fomewhat nearer than it was at firft, ib
that the guaces which were at the Extremities of the
longer Di^eter, are the Extremities of the íhorter one ;
and thus the Circumference of the Bell changes its Figure
by Turns, all the time it is founding.

2'o. It will be veryci^fy for any one to believ*e what is 20. bí Proof
now faid, if he obferves, that in laying his Hand upon a
large Bclljuñ when the Clapper ftrikes againft it, he will
íeel a manifeft Numnefs.

21. If the Bell be very fmall, as the Trembling is eall- zr. wiy
ly ñopt by putting our Hand to ir, fo oughc the Sound “

to ceafe alfo. And indeed there are very fmall Bells, which
if they be buc very lightly ñruck, will found for a long time ; fcund.
bnt if we lay our Hand upon them as foon as tfcy are
ñruck, their Sound will immediately ceafe.

22. But che Sound of agreatBel) isnocfo eafilyftopped n. W'íjkSí
by laying our Hand upon it, becaufe it has .more Motion,

and becaufe it - can transfer fuch a fmall Part of its Í„ 7 f, elfiiy

Motion to the Hand, and referve enough to make it be

heard.

23. The Sound raifed by ftriking a Piece of Wood, or •*

in general, any hard founding Body, coníiñs in a Trem-

bling, like that of a Bell, which is owing to its Sprin-’y?>'»cí'.
ginefs.

24- Wherefore Bodies, which have not this Property of a-t. wh
Springiiiefs, have only a very low and imperfedl Sound

And t/e Somd,

I8S

ROHAULÜU SysTE

M

Partí.

And this is the Reafon why 'Lead and Clay, when the^tí
llruck againftj have fcarce any Sound/

rml'f^íoti '*■ very dif.

mofíhe Air fi^ulc to determine what forc of Motion it is in the Air
it is in which which produccs in US the Seníátion of Sound ; for it i¡
Smniim- eyjdenf^ I tijis Motion of the Air muíl neceíTarily
be fuchj as the Trembling of the Sounding Bodies is ca-
pable of producing in it : that isj the Air ought to treni-
• ble, and bubble, and alfo by riíing and falling, to divide
it felf into an infinitg.number of very fmall Particles,
which by trembling and ftriking againíl one another,
tnuíl have a very quick Motion; fo that the Air muft
be fomething like a Liquor that limpers and does not
quite boil. This is confirmed by what we fee of a Mo-
tion very like this in a large Tub of Water, by moving
a Stick backwards and forwards in it very quick ; for
this Motion t)f the Stick is very like that of the Strings
of a Luce, only thefe are inuch larger and tlje ojíler
flower. W

aS. A vifihie 20. W e may be certain of this Motion or Trembling of
Demonjirati- ^í^ir, if we coníider that the founding Bod.Y> ought to
impreís the fame fort of Motion upon it, mat it does
upon other Liquors. Thus, if a Glals be half full of
Water, and we^make it found in the Manner before-
mentioned, by moving our Frng^along the upper Edge
of it,- it muít without doubt íliaKe the Air as it does
the Water; ^ now we fee the Water tremble and boil,
and alfo by jumping out, batter and brcak it felf in
fuch a manner, that a great mány fmall Drops fly a
good way out of the Glafs. Whence we muft con-
clude, that the Air has the fame Sort of Trembling or
Boiling.

y

I . This Motion of the ÍAir) For
the Parts of che íbunding Body go-
ing and coming by íurns, chruJftand
drive forward as. they go chofe
Parts of che Air which are nexc
theni) and by prefíing upon them,
condenfe them j chén by recurning,
they permic the Parts thus compreíl
fedj to fpread and dilate themíelves
again. Thoíe Parts of the Air there-
fore which are nexc to the íbunding
•Bodys go and come by turns agrec-
ably to che cremulous Agitación pf
che Parts of che íbunding Body i
and in che fame manner as che
Parts of Chat Body agítate thefe Parts

of che Aíra do theíé, being agitated
wich the íame Sort of Tremblingí,
agítate chofe Parts that are nexc
them 3 and thefe in like manner a-
gírate thofe beyond them, &c, This
being allowedj the manner how the
Pulfes are propflgated along, andalJ
che other Phsnomcna of Sounds, are
very advancageoufly explaíned. See
Netot. PhUofoph, Princip» Maíhemau
Sooí* II. Prop. 43, é^c.

2. Noto we fee the VlAater trmhlt)
You may fee a Cafe of this Experi-
menc very well worch obferving in
the Notes on the An^ of this
Chtíptfr» I

Chap. 26. of Natural Philosophy.

27. AftQí having fufficiently íhown the Motion of the a?- ^neme
Air, whicli is neceirary to make us hear airy Sound : Ic

is eafy to conceive that the' Air in paffing by fome hard ¿hkh is ’
and immoveable Bodies, ’may move it felf fometimes in rnadebyhhw-
fuch a manner. Thus, when we whiftle, by blowing Bf//»/
into the Holhw of a Key, it happens, that the Air which l{_ey.
entera in, filis one half of the Hole, and the Air which
comes out filis the other half, and thefe two Pareéis of
Air fliding by one another with contrary Motions ,• a
great roany of their Parts muíl neceíTariíy be made to
turn round and to tretnble, and the whole Air which
is betwixt him that whiíües and him that hears muíl al-
io be made to turn round and to tremble.

28. Wemay obferve here, that there are Bctdies, which h«w the
are opened by Fits to let the Air through, and which

by this means caufe us to hear a particular Sound, which
is alfo a very confiderable one. Of this Sort are the
Ra^s of Pipes which compofe an Organ, or the fingle
Pi* of a Bag-pipe. . Thefe Bodies themlelves are not mo-
ved in order to produce Sound ; but the Air being firíl
piit into ^tion, endeavours to pafs through them, but is
forced to go out trembling, and fo impreíTes on the reft
ofthe Air the fame Sort of Tremblings as the Strings of a
Violin do, and fo caufes us to hear a Harmony, the Mo-
tions of which are T^mbling.

29. And in the fame manner is the Voice of Anmah u»» tU
formed : For there is a ímall Valve at the End of the l„Tis¡ífir-'
Trachea, which performs the Office of the Valves of the med-
Tubes which compofe an Organ which Valve we can

centrad as we pleafe, and let the Air out of the Lungs
by Fits. And becaufe this Valve for the molí: pare con-
tinúes open, therefore the Air in Reípiration comes out
commonly without any trembling, . and confequently
without making any Noife.

30. It would be too tedious to explain particularly all 50.
the different Manners in which Sound is produced. But

becaufe there is fo'mething fingular in che Sound of aCaw- '¡^ ¡“¡'"ff"
im when it is difeharged, becaufe che Fíame feems to ciw^cd,
give but one and not a repeated Shakc to the Air, there-
fore it may be W'orth while to explain how ílich a prodi-
gious Noife is made. It is fo be obferved then, that the
Gun-powder, when ic takes Fire, r is fo extraordinarily
dilaced, as to take up above a Thoufand times the Space

I. A /o exlraordinárÜy diUtec)) I on, See '.be Notes on Part, IIÍ.

For the tme Reafun of tüis Dilatad - 1 Ckap. 9. An. i 5,

that

xgo RO H JULT’s Systzu Partí,

that it did before j fo tha: it drives before it '.every Way
all the Parts of the groflér Air which was in chis Space,
and thefe Parts can find no.where cogo, but by préffing
upon other Parts, and driving them on likewife j and at
the fame time they íquecze ou: the fubcile Matter which
mixing with the Powder, compofe that fenfible Mafs
which we cali Fíame. Henee it follows, that there is in
the Air two contrary Motions ; the one of which gachers
togecher and unices the moft fubtile Parts, and the othei
difperfes the groíTer ones. And chis would be done in a
Momenc, but that- the groffer Air which is condenfed all
round, has. a Tendency to return into that Place out
of which it was driven, and cowards which, after the Vio-
lence of the Fíame is o ver, its own VVeight forces it, and
that with fuch an Impetus, that it becomes more denle
than it ordinarily is ; whence it will be refleóted again all
round, or condenfed anew; becaufe being rarefyed again,
it returns to che Place which it had quitced ; and thff it
quits and takes again the fame Place feveral times ruc-
ceílivelyi and this is the Reaíon of that fhorc Continuante
of the Noife of a Cannon when it is difchargedjfi
ji. the 51. Howevcr it is to be obferved, that the Ear inay
the^'Tótmf be fo fti'ongly moved, that it may continué to

mntiimesim- tremblc fome Ihort time after the Air has done tretn-
ger fometimes bling j and for this Reafon, Senfation of Sound
stwíi;/ feif. fometiraes continué after the Agitación wichout is

ceafed.

31. whythe Becaufe che trembling Motion of the Air in which
Fiajh ofa Sound coníifts, is communicaced gradually, fo that it
f^’béfiruhe thofe Parts which are near the founding Body

SomJ tí fooner than thofe that are further off, the Sound muft
/teard. neceíTarily cake up fome time in going along : And fo we
find by Experience, thatif a Cannon be difeharged at two
or chree Miles diftance from us,we fee the Flaflr fome time
before we hear the Noife.

33. Whythe 5^. And becaufe the Motion which is impreíTed by
weuhef^Thl founding Body upon the Air clofe by it, is tranf-
further ne ferred from one Part of the Air to ■ another fucceffively,
fronffhT‘ always paffes from a lefs Quantity to a greater, in
%¡»diii¿ Bo- proporción to its Diftance from the founding Body ;

therefore near the founding Body, there muft always
be more Motion in a given Quantity of Air, than there
is at a greater Diftance ; fo that the Sound ought to grow
weaker as it is further from the founding Body.

Chap. 26^ of N ATURAL Philosophy.'

34. The Propagation of Sound may very well be ^ com- 54..
pared with Circles made in the Water, by throwing a

b'Eone into it. And as thofe which are made in a run'ning mn^“o„ght
Stream, extend themfelves further towards the lower cban be bceri
towards the upper Part of the River, becaufe the-^^""
whole Water in which they are formed" carries them in- «.
tire that Way ; So likewife may we conceive, that if the ^

Wind carries the Air towards one certain Place, the trem-
bling Motion in which Sound confifts, wili fooner go this
Way than the contrary. Thus we fincf by Experience,
that we hear the Sound of a Cannon, and in general all
ocher Sounds, ^ fooner with the Wind than againft it. And
it may happen, that the Air may be moved fo quick, that
its Parts may flee from us as faft as the Sound goes, and
fo we may not hear it at all.

35. Becaufe Sound is propagatedevery Way, as it were jr-
from the Center to the Superficies of a Sphere, it may
fsliappen, that the Parts of the Air which would com-
n^icatetheir Motion to fuchasareatagreater Diftance,
may meet fome hard Body which they cannot íhake ;
and this may caufe them fome Way to be refleíled back
again, affi^ make them communicate their Motion again to
thofe Parts from which they receivedit, and thefe to others;

fo that there will be a new Trembling of the Air inílead
of that which began firft, and hath already ceafed for
fome Time ; Confeípently we may hear again the fame
Sound which we heard at firíl,- and this redoubled Sound
is what we cali an Echo.

16. If the Sound meets with feveral Bodies at different 36. Jimaa
Diftances, which are capable of refledling it back again,-
if that wliich returns from the mofl: diñant Place ñrikes jfoLnfivJjil
upon the Ear, after the Impreffion of the former is r»®"-

I. Compared mth Circles made in j
the Water) If the Water be put in
Motion, by throwing in a Scone, ’
or by» moving our Fingir or a Stick
backward and forward in ic, the
Waves will immediately furround
our Finger j and if during the Agi-
lation it be carried ftreight forward
towards any Part withouc turning ic
round, yec thefe Waves, as if they
were concentrick Gíreles, v/ill be e-
flually propagated every Way ; which
Comparifon does very properly íhow
üs, that the tremulous Motion of che
Air oughc co be propagated noc on-
íy che fame way that every one of
(lie J^íjríiUes qí tile founding BQtlys

fuch as the Scrings of a Violín, are
I agitaced , but aab to be propagated
' in a Gírele all Ways from che found-
ing Body as the commonGencer.

2 . Sooner with the Wind than a~
gainjlit) The Gentlemen at Florerua
choughc they had found by certain
Experimenes, thac Sound is propa-
gated with the fame Gelerity againft
the Wind, as with ic, though much
more fainc. Exper, Acad, del Ci-
mérito^ p. 140, But the induftiious
Mr. Derham found it otherwife ia
Experiments madeac a much greater
diftance. Seethe Philofophicallranf-
aílionsi Nnmb. 313 .

192 ROHAÜLT^s System Partí,

quite gone ofí, ic muíl in its Turn produce a new
Senfation of Sound. Whence ic is evidenr, that we may
ineec wich Echo’s whích repeac. che fame Word feveral
times over.

■^•j.vyhyhe According to the loclination with which the Ait
doéfrtuaZ ftnkes upou the Bodies which reñeót the Sound, ought the
ways hiar the Refledion to be on the one Side or onthe other, which
^¿'ch Reaíbn why there are fome Echo’s where he who,

fpeaks doesnoc^hear the Words that are repeated, when
others who are atfomeDiliance from him can hearthem
repeated diftindtiy.

38 . m^í 38. As to the Difference of Sounds that we meet wich,
‘spedí^ef which conítituccs the different Species of them^ as Fíats
SoimJ cmfiji and Sharp-, the mufical Inñruments fufficiently íhowus,
that rhey confifl: in the diíFerent Mocion both of the
íbunding Body, and of the Air which is agitated by it.
For the more the Strings .of a Lute are Jlrain'ed, the
Jbarper the S lund is ,• and on the contrary, the loofer áe
Strings are, the mote flat is che Sound. Now it is m-
tain, that the more a Scring is ílretched, the fwifter and
more frequentis the Motiou which ic impreffeson the Air;
whence ic follows, that a fiarp Sound conftxcs in the
^icknefs and in the fudden Reiteration of the Motion upm
•which the Somtd depende, and a fiat Sound confifts in the
Slownefs.

39. When two founding Bodiefitrike upon the Air at
the fame time, they muft imprefs fuch a Motion upon ir,
as is compounded of the two Motions which would be
caufed, if they aóted upon ic fcparately and confequeiitly
the Air ought to put the Orgast of Hearing inco luch a
Sort of trembling Motion, as may raife a Senfation com-
pofed of each of the Senfations which the Bodies would
raife feparately.

40. And if the Mocion of thefe two íbunding Bodies
do fo exadtly agree, that che Tremblings which they caufe
in the Air in a given Time are commtnfurable, that is,
ac the fame time that the one ítrikes the Air, the other
ftrikes ic alfo, or at leañ, that they ílrike together every
íecond or third Stroke,- then the Ear will be lo uniform-
ly ílruck upon, and in fuch Meafure, that it will perceive
the Diftance, and be pleafed with the Cadenee 3 and iii
the Strokes being thus commenfurable very probably con-
fifti thofe Cosicords which Muficians cali an Vnifon atid 05av¡,
« Fifth and a Third,

59 , How Je-
•veral Sounds
may be hcará
together»

40 . yyhat
Concords con-
fífi in.

Chap. 26. of Natural Philosophv.

41. On the contrary, if the Tremblings impreíléd on
the Air bythe Sounding Bodies be incommenfurablcj thac
isj if they do not agree in Time ñor ftrike togecher,- we
muft perceive the Inequalicy of the Sound j and becaufe
they do not move the Ear uniformly, tijey cannot pro-
duce any Harmony , and in the Strokes being thus ih-
commenfurable, confiíls very probably the Tones which
Muíicians cali Difcords.

42. From what has been faid concerning the Motion
impreíTed on the Air by founding Bodiesj, fome Perfons
perhaps may be apt to think that thofe impreíTed by the
Strings ofa Lute are not equal ,• but quicker at firft, and flow-
er as the Motion ceafesj butitis not very diíBcuIt to íhow
that the contrary is true, if we obfervejthat the Motion of the
String when it almofi: ceafes to be agitatedj, may be made up
by the Shortnefs of the Way that it has to go : So that it
takes up neither more norlefs Time inmaking its firft and
iMgeft Vibrationsjthan it doesin making itslaft andftiorteft.
^3. There muft indeed be fomePains requifiteto prove
theTruth of this by Experiments : For it is impoffible to
doit by the Strings of a Lute, becaufe of the Tmall Time
that thej^ke to make feveral hundred Vibrations in. But
becaufe the Motion we are fpeaking of is very like that
of a Wéight hanging in the Air at the E 7 id of a Strings we
may imagine, that what we obferved of the Motion of the
one, may be equall/'ípplied to the other ; Now we find
by Experience, that if this Weight be drawri from the
Perpendicular, and then let go, fo as it may fwing freely,
all the Vibrations till it ceafes to move at all, will be made
in the fame Time. For if we will be at the Trouble to
count how many Pulfes of the Artery there are in the
firft twenty Vibrations fuppofe, we ftiall find as many in the
twenty following Ones, or in any other Twenty, which
you will: Now from this fingle Experiment we may con-
clude that every Vibration oí the String of an Inftrument
is made in the fame Time, and that the Laft take up no moré
than the Firft. And becaufe this Experiment is very eafy
to make, and is a curious one, and may ferve as a Prin-
cipie from whence many important Conclufions in Mu-
fick may be drawn j it is worth any one’s whüe to be at
the Pains to obferve the Motions of thefe Pendulums, and
to put feveral of them in Motion together. For we íhall
then fee, that thofe which are of an equal Length, and
alike in every other rcfpeíft, will perfprm their Vibrari-
ons in the fame Time 3 and that thofe which are of dif-
ferent Lengths, require different Times;> vía. the Shorter,

O the

193

41 . my
fome Sounds
are Dijeerds-

42. Thnt íhi
laji l^ihrati-
ons tf the
String of a
Lnte do nót
take np moré
Time than
thefirji.

4^. Oflhs
¡i-híiori of
Pendulums,

1^4 RO H AU LT’s System Partí,

the lefs Time, fo that their Vibration,'; will be to eacli
other J in a reciprocal Proportion of the Square Root of
their Lengchs ; and thus what we have faid of the com-
menfurability of Soutids, and the Concords of Mufick, is
confirmed. *

4+. rvh.nce 44. From henee we may alfo clearly apprehend how
‘sfnsüfvmes Sorts of Voices are made, and why the fame

arífi,a«T" Mouth may caufe by turns a íbarpand a fíat Sound. The
why the Volees Reafon of which is, that the Efiglottis which is placed at
"Li^’^neTaiiy thi'ough which wc breath, and which

Jharferthm opcns to give a Paífage for the Air in order to form the
thofe o/¿nan Voicc, may be lifted up and let down at pleafure, that
is, fo as fometimes to be altogether and from its
Roots open, or íliut, and fometimes in Part only. Now
that which can be lifted up in fuch a manner as this, by
T urns, and as it were with a trembling Motion, to let the
Air out with the fame fort of Motion, refcmbles a Pra-
duluvi-^ whence it follows, that the Tremblings of Jie
Voice muíl be fo much the quicker, che lefs the
glottis which regulates che Motion, is lifted up, and on
the contrary, chey are the floweft that can be,^7hen the
Epglotíis is at liberty to lift it felf quite up. Üpon this
Flexilenefs of the Epiglottis depends all the Variety of
Tones of the Voice ; for the Air which comes out of
the Lungs being differently agicatec^;iccording to the dif-
ferenc Pofition of the Epiglottis, imprelTes the Motion
it received as it carne out, upon the external Air, which
ílriking che Ear differently is the Caufe of all that Diver-
ficy which we obferye in Sounds. And becaufe Children
have gcnerally all the Parts of their Bodies proportioned

I. In a reciprocal Proportion) Here
the Numher oí Vibratíons in a given
Time are compared with each other.
Buc if the Times of the Vibratíons
be compared together (which is the
beiter VVay) then we miiftfay, that
the Vibratíons are to each other, as
the ítjuare Roots of their Lengths
direlily. As may be thus demon-
íirated. We fuppoíé that the Ac-
celeration of heavy Bodies in falling
is íüch, that the Spaces thíw run
through, are as the Squares of their
Times (which íhali be demonífra-
ted in its proper Place. Sce the
Notes on Part II. Chap, xicviii, ./írr.
16.) then if vve imagine llmilar Ares
of unequal Circles to confifl: of an
infinite Number oí Sides of finálar

Polygons, and that th^ are in the
íáme Feficion with reípedl to the
Earth ; then it is evident, that the
Square. Roots of the Arches, or of
the Spaces run through, and for the
lime Reafon, their lladius's or the
Length of the Strings, will repte-
feot the Tinies of the Delcent of
Pendulums ; and becaufe the impe/Ȓ
or Velocity inafeendingi is evident.
ly deftroyed equally in the fame
manner, and in the fame tíme as it
was acquired in defeending ; thete-
tote the whole Vibratíons of thefe
Bodies muíf neceffarily have the fame
Prnporíion to each other, as the
Square Roots of the I.engths of the
Strings. See the Notes on Part II,
Chap, x.’tyiii, .,Art. J S,

Cha.p. 20 . of Natural Philosophy. 195

to their Bignefs, and confequently their Epiglotcis, lefs
than in grown Perfons, therefore the Voice is generally
fharper.

íj.5. And altogether as eafy is it to account íor an Expe- -the Rea.
timent which at firíl Sight has furprized a great many A” ¿
Perfons ; which is, thac if cwo Strings of the íame Ente,
or of difFerent Lutes thac are near one another, be Uni^ Cmctrds:
foKS, we cannot move the one, ' but the other will found
alfo, at leaíl it will tremble • whereas ic will not tretnble
at all, if we move any other String near it, which is noc
a Concord. Now the Reafon of chis Experiment is, thaC
the Strings which are Concords, are capable of the fame
Vibrations fo that che Air which is put in Motion by
the one, can very conveniently communicate its Vibra-
tions. to the other j which cannot be in two Strings thac
are noC Unifofis; for there is no Agreement in them, be-
¿:aufe the Air which is put in Motion by the one,

^es not find the other at all difpofed to receive its
Motion ,• and every^troke, except the Firftj is out of
Time, fo thac by noc agreeing they deftroy each ocher’s
Motid^

5|.á. This Experiment has raifed the Admiración of ma- 4^- T&Ame
ny Perfons for a long time, and fome have undercaken to
account for it, by faying, that there is a Sympathy be- other •BeiUts,
tween the two Scrii'-jS but, befide that this is only a Way
of fpeaking, we may obferve, that the Difpofition which
a Body has to move, when the Air is fhaken by another
Body, 2 is to be found in other Thirigs as well as in the
Strings of a Lute, or other Mufical Inftrument ; This I
have experienced in the late Wats, when I have obfer-
ved the Glafs-Windows to tremble very fenfibly upórt
the beating of a certain D’rum, and at the fame time would
not tremble at all upon the beating of others which were
much louder-

1. S/if the other xolll Jonnd '
So likewiíe if cwo Glaíles, ty put-
ting in a proper Quantícy of Wa-
ter) bemade Uniíonsj che preffing
our Finger hard upon che Edge and
moving ic round cicher of chem»
■will make the Water in che other
curh and dance abour.

2» lí to he found in other 2 hings)
Thus Mr. Hoyle relates concerning a

fick Man that had his Left Hand
ene ofF i upon the difeharging , of
Cannons, he thoughc himfelf almoíl
íhatcered and torn to pieces > and
of another, that upon feraping a
piece of Iron v/ith a Knifes he
cuuid not hold his Water s and of
a Third^ thac upon tearing chick
Paper his Gums would bleed. See
his af langidd Motion,

1^6

RO H AU L'J’’ s System Partí

47- i> 47. To thefe Sort of Motions, I conceive we may,af-
ths au¡e the Caule of a cercain Sbivering-, which we fomé-

that fhiver--

ing which we times feel all over our Body,

feel upon
hearing a

Trnmpet.

48. Hovo iee
rendir our
fehes atteri’
tive^ fo as
to hear
Sojinds di"
fiinñiy.

and which reaches even to

our very Heartj when we hear the Sound of a Trum-
peCj or fuch kind of ínñrumenc j For it may be that tlie
tílood Í3 fo difpofed, as to yield eafily to the trembling
of the Air.

48. And becaufe the Membrane of the Ear, which is
moved by the Agication of the external Air, the dif-
ferent fhaking of which caufes difFerent Motions in the
Capülaments of the Nerves of the Ear, is fomething
like the Parchmenc of a Drum fand is therefore by fome
called the Drum of the Eár I am of Opinión^ that it is
capable of being more or lefs íhaked, according as iris
more or leís ftretched. Wherefore lean eafily perfuade
my felf, that we fometimes ftretch or loofen ir, in or-
der to receive the Impreífion of the Sound more {en¡
b!y, and to malee it the better agree with the Motionil
the external Air: So that Attentioj^onfiñs in nothing e*
butin a due ftretching or loolening this Membrane j and
keeping it in that Pofition in which it will bef^'í-eceive
the Impreífion and Mótion which the Sound gives to«the
external Air.

C H A P. XXVII.

Of Light and Colours, and ofTranfparency, and
■ . ^ . ■ -^Opakenefs,

Sel'jffthe^ T F in any Thing ExaAnefs be required in the Meaning
wirdiught of Words, in order not to be furprized by any Equi-
ond Ceimrs. vocation, it is principally in this of Ught and Colom,
.which are commonly ufedto fignify very diííerent Things,
and generally confounded-.by moft Men. Firft then it is
to be obferved j that ás we have given the Ñame Fam to
the Senfation, which is raifed in us by a Needle when it
pri.cks us; fo likewife have we given the Ñame Light to
that Senfation which we have, upon looking on the Sun
or a. Fíame, and that of Co¿our to the Senfation raifed in
US by di\''erfe Objeíts which we cali coloured • thus in
particular, we give the Ñames of a White Colour and a
Green Colour to the Senfations which í'waw or Grafs ufu-
ally píoduce in US. "

3. Se-

Chap. 27A/’ N ATURAL Fhilosophy, 197

2. Secondly, By thefe Woxás Light and Colour, we i- Amther
alfo underñand, that on the Part of the external ' Ob-

jeíls which is the Cauíe of exciting in us the forcmenti- and atoar.
oned Senfations ; Thus by the Light of the Fíame, we
mean fomethi'ng, I know not what, which occafions the
Senfation of Light to be excited in us ,• and by the Wbite-
nefs of the Snow, W'e underftand fome other Thing, I
know not what, that is the Occafion of our having the
Senfation of Whitetiefi.

3, And becaufe the Objedts which w'e cali luminous, Athhd
fuch as the Sun or a Fíame, do not aíFeíf ourEyes im-
mediately, butad: bythe Interpofition of fome interveen-

ing Bodies, fuch as Air or Water or Glafs ,■ yet that which
is irapreíTed on thefe Médiums, whatever it be, is called
light alfo, but Seiondary or Dermaíh'e, to diftinguilli it
from that which is in the luminous Objeds which is cal-
led original or innato.

We cali Chole Bodies Tranfpare?itj through which
iSiinous Bodies ad: upon our Eyes to raife the Senfa- '"í "/
tion of Light, and through which we can alfo fee
lours. ,^d we cali thofe Bodies Opake which interrupt Opate,
the Adion of luminous or coloured Bodies, or through
which we cannotfee either Light or Colours.

5. I do not pretend to declare what Light and Colours j.. xhattht

are in the firfl. Senfe^^f the Words, but leave it to eve- ^‘pf^tíonof
ry one to malee thefn clear to himfelf by his own Expe- f"-

rience j for I think it as impoffible to give anoilier Per- defcrikdj
fon a true Notion of that particular Senfation that we

have of Colours, as it is to give it to one that is bom
blind.

6 . However, I may venture to afSrm, that as it of- natom
ten happens that the lame Food may at the fame time ‘”>d ¡he famc
raife dififerent Taftes in two difFerent Períons-3 ib it may

alfo happen, that two Perfons looking in the fame man- ccjfariiy ‘/aire
ner upon the fame Objed, may have very different Sen- the fame Sen-
fations j and I am the more perfwaded of this, becaufe diff/emp/r.

I have experienced it in a particular manner my felf. /««,,

For when I had once quite tired and weakned my right
Eye by looking intently for above twelve Hours toge-
ther through a perfpedive Glals on a Bátele betwixt two
Armies, within a League of me ; I found my Sight fo
afFeded afterwards, that when I looked upon Yellow Ob-
jeds with my right Eye, they did not appear to me as
fhey ufed to do, ñor as they now do to my left Eye :

And, which is very remarkable, I do not find the fame
Pifference in all Colours but only in fome 3 as for in-
O 3 dance

1^8 ROH AUL't’s System Ü Partí,

flanee in Green, which appears to me to come near to
a Blue:, when I look on ic with my righc Eye. This Ex-
perience makes me believcj that there may be fome Men
born with that Difpoficianj which I at prefent have in
one of my Eyes, and which may continué all their
LiveSj and perhaps there are others whofe Eyes are of
the fame Difpofition with my other Eye. However, it
is impoffible, either for themfelves or any other Perfons
to perceive it^ becaufe every Body accuñom themfelves
to cali the Senfation which a certain Objedt produces
in him, by that Ñame which it ufually goes by j which
yec being common to the different Senfations that every
one may poíTibly have, is not the lefs ambiguous.

7. Ariftotie’í 7. Before I come to that Enquiry which I defign, m.

Light is, and what the Colour of Objedts is, which
is the principal Defign of this Difeourfe ; I obferve, that
Ariflotle has treated of the íame Subjeétj in the yth
Chapter of his Second Book Concerning the Soul j wherft
after having faid, that Colours depend upon Light in ordff
to their being feen, he concludes, that thefe two Qua-
lities ought to be explained together. And iu ^rder to
determine what Light is^ he fuppofes that fome Boíiies are
tra7if¿arent¡ fuch as Air, Water, Ice, Glafs, and fuch like.
And becaufe we cannot fee through any of thefe Bodies in
the Night, he íáySj that then they are in Poever only trasf-
yarent-) and that in the Day-time*f/3ey hecome añually
iranfparent j And becaufe it is Light alone that can bring
this Power into Jíi, he concludes^ that Light ¡s the Añ
ef a tranjparént Body as tranfparent.

8. TrjhCpL 8, As to Colour^ he obferves, that fince the Ob-
which it is, does not apply it felf immediately to
our Eyes, in order to laife any Senfation in qs, it muíl:
firfl mpve the Médium which is betwixt that and us,’ and
becaufe it cannot be perceived through Opake Bodies,
ñor can it be feen through thofe that are ofs¿y trarsfpa-
rent in Power, he concludes, that Colour is that which
vioves Bodies which are aSiuaUy tranfparerst.

5. That he cj. Tbough Ariflotle in the forecited Chapter, has not
^timtiyl'f'' ^^®rched this Matter to the Bottom, yet he.afErms, that
plained what he has fufficiendy explained what Light, and Colour, and
aad Travfparency are, and imploys almoft all the remaining
fiican aie. Difcourfc, in refutirtg the Opinions of fome

Philofophers that were before him. However he adds,
that Light is not Pire, ñor a Body proceeding from a
Luminous Body, and paííing through a tranfparent one ;
|cuc only the Prefence of Pire, or any other luminous Body

with

199

Chap. 27. <3/ N A TUR AL PHILOSOPHy.

with the tranfpai^it Body. But upon confidering this O-
pinion, I fee no r'eafon to be fully Tatisíied with it, as if
it could not Be carried any furcher than Árifiotle has
done, or at leaftj that it cannot be more diñindHy ex-
plained. For it is certain, we are ñill ac a lofs co find
out more particularly what the Nature of tranfparent
Bodies, and alfo what the Nature of luminous Bodies is j
and further how the Frefence of the Latter operates 07t
the other, to bring its Power mto ASI j and lañ of all,
what that is which moves a Body that is añually tra^if-
paretit.

10 . This fome of thé Commentators upon Arifiotle lo.inatthe
have acknowledged ; and though they might have had

fome Light from what he has faíd in his Problems, and is concerning

particularly from ' the 61& oí úíq Ehventh SeSion i yet^'¿*‘‘”“<

they have either overlook’d what he has faid in this

Place, or at leaft not rightly underflanding him, they

fove advauced fomething which it does not appear that

mifiotle ever thought of, viz. that Light and Colours in

the Objeíts which we cali luminous or coloured, are

Qualitj^ exaftly like thofe Senfations which they occa-

fion in US and (as fome of them contend) they arife

alfo from a Mixture of Hot and Coid, of Dry and

Moift. And for Proof of this (befides their thinking,

that they have Arilhtk on their Side) they aííirm, that

it would be impollíole for luminous or coloured Bodies

to caufe thofe Senfations in us which we feel, if there

were not in them fomething very like what they caufe

US to fed j for, fay they, nothing can give what it has

not.

11. But, befides that Arifiotle has faid nothing poli- x\.Th,uthty
tively concerning what they have advanced, Authority ¿«f
ftands for nothing, when we are inquiring after Reafons

only. And as to what they alledge, it will appear to be '
only a mere Sophifm, if we refled ever fo little up-
on the Pain which we feel when we are pricked by a
Needle ,• for this íliows us, that it is not at all irnpoffi-
ble for an Object to be able to excite in us a Senfati-
on which it felf has nothing of. And chis is ftill fur-
ther confirmed from henee, that two Men may fee the
fame Objed diíFeren'tly, as was before obferved, I roy *
felf feeiug Yellow»'differently v/ith my two Eyes.

T, The6\fi of the Eleventh Jyf^iion)
Where, aher having propufed this
Qüeñion. Why toe canuot fee phrough
an 0¿ah "Body* He argües very

04

much for the Propagación of l.ighc lu
ñreighi Lines.- Üee the hJotes en the
Inter Part of the ij ^rt, of ihií
Chnpter.

iz- Buc

V

200

l2. Thilt it
is not trae.

13. TheM-
furdity (¡f the
Opifiio» of
Jome of the
Anítocelians.

14 . u/á Com-
purifoti of the
Senfatiou of

X.ight Toith
that oj Saín,

RO H AULT’s System Partí.

12. But Chat which moft evidently that it is not

at all neceíTary there iliould be any Refetnblance between
the Quality of che Objeólj and the Senfation it excites,
is chis j that we certainly have very ftrong Senfacions of
Red, and Ycllow, and Blue, and all other Sores of Co-
lonrs, upon loofcing through a Triangular Glafs Frifin, in
which no one ever fufpeded that there was apy Thing like
the Senfation which it raifes in us.

13. That which others of themíáy concerning the 0 _-

riginal of Colours is ftill more abfurd. For what Con-
nexion is there becwixc the Ideas we have of Hoc and
Coid, Dry and Moiffc, and chofe which they fuppofe us
to have of Colours ; If what they fay were true, it would
from henee follov/, that the fame Objcvll: ought to have
as much Variety of Appearances to che Eyes, as it raifes
differenc Senfations to the Touch 3 which does not agree
with Experience ; On the concrary, there are fome Bo-
dies, fuch as poliíhed Steel, and Lobílers, which whe|
heated by the Fire, acquire a certain Colour 3 but whwi
made coid by dipping them in Water, they do not alter
their Colour. ij.

14. Leaving therefore the Opinión of Arijotle and
his Followers, concerning Lighc and Colours, lee us now
confider what Partwé are tq calce upon chis Subjedl. And
Firíl, Since we have no Reafon to that the Light of
luminous Bodies is any Thing elfe but the Fower which
they have to produce in us that very clear and hright Senfa-
tion which we have when they are befare us ^ Why may
we not compare this Power with that which a Needle
has to caufe Pain in üs ? Since then che Senfation which
a Needle railes in us, fuppofes only that we are íenlitive
Creaturesj and noching more is required inthe Needle but
its Figure and Hardneís, which are- alone fufficienc to
cauíé a Diviñon in the Pare to which it is applied : So like-
wife it is reafonable to think, that the Senfation of Light de-
pends upon chis, that we are byNature made capableof
this Sort of Senfation ; and that there is in the Pores of
rranfparent Bodies, a Matter fine enough to penetrare even
Glafs, and yetar the fame time ftrong enough to fliake the
fmall Capülaments of che Nerves which are at the Bottotn
bf the Eye. Further, as there muft befóme Agentto
pufli the Needle into US, folikewife inuft we think, that
this Matter is pullrcd by the luminous Bodies, befóte it
can malee any ímpreffion on che Orga 7 s of Sight.

ly. Thqs

Ghap. 27- of Natural Philosophy, 2Gr

15. Thus > Original Light confifts m a certain Motion ir-
of the Taris of luminous Bodies whereby they are capable °of ¿llTTnd
of pufliing every Way the íubtil Matter which filis the Tranffaremy,
Pores of tranfparent Bodies j and the Ejjence of fecondar y
or derived Light confiñs in the Dijpoftion or Tendency of ■'
this Matter to recede from the Cefiter of the luminous Body
in a flreight Light. Whence ic is eafy to infer, that the

Forra

1. Original Light-'—Secondary or
(¡erhed Light) Original Light con-
lifts intirely in a particular Motion
pf che Particles of the luminous Bo-
dy i noc whereby they puíh forward
that fiíSbitious Matrer which Cartes
imagined the Pores of tranfparent
Bodies to be filled with 5 buc where-
by they íhake ofF lome very fmall
Particles frora the luminous Bodya
ijthich are fent forth all Ways with
greac Forcé : And Secondary
Sikerivativc Light conCfls, noc in
the Dífpoíition) but in the real Mo-
íion of thofe Particles receding eve-
ry way che luminous Body in
ílreighc Lines with incredible Swift-
nefs. For if Light confiñed only
in Pregare, ic oughc to be propaga-
ted to all Diñances in a Moment of
Time j which it certainly ,J'^oc (See
the Notes on ^rt, 30. Andic

would noc be propagaced in üreight
Lines, buc it would perperually run
inupon the Shadow. Por PrefJJon
or Motion cannot be propagated in a
Finid in right Lines beyond art Objla^
clcy which pops part of the Motion,
but roill hcnd and fpread every Way
inio the (jfiípfcent Médium, xohhh lies
beyond the Objiacle.* Gravity tends
ííownwardss but the PreffitYe of Wa-
ter arifingfrom Gravity^ tends every
way with c^ual Forcé, and is propa-
gated as readily, and with as much
Forcé fideways as downwards, and
throHgh crooked Pnjfages as throngh
firelght 07 ies. The través o« the Sur-
face of Jiagnating Water, pnjjing by
the JJdes (f a hroad Ohjiacle which
fops part of thcm, bcnd afterwards,
and dilate themfelves gradually into
the ^iiiet Water befn/id the Objiac/e,
The WaveSi P 7 :lfes or Vibrations of
the ylir, wherein Sounds confifi, bend
manifefily, tbongh not fo much as the
JFaves of Water-— ^nd Snunds are
progagated as rea&ily through crooked
Pipes as throngh fireight otjcs» "But
Light is never known to follow crook-
ed PaffageSi ^or to bend into the

t

Shadow» Newt. Gpticks pag. 537.
Rays of Light therefore muft be'
fmall Corpuícles íent forth from lu-
minous Bodies with a very greac ce-
lerity. For fuch forc of Corpuícles
(contrary to che Preílion of Motion
propagated in a Fluid) oughc to be
cranfmitced through uniform Médi-
ums or void Spaces in ílreighc Lines,
withouc bending into the Shadow 5
as we íée the Rays of Light are
cranfmitted.

Concerning that Forcé by which
thefe Corpuícles are fent forth with
fuch incredible celericy, that they
are carried above 7000000 of
Miles in a Minute (See the Nor-es on
Art, 30. belowF) che admirable Per'*
fon before-ciced fpeaks thus. Thofe
Bodies which are of the fatne kindand
have the famc Vcrtne, the finaller they
are, the jlronger is their attrafíive
Forcé in Proportio» to their Bignefs.
(See the Notes on Chap. xi. Art. i f .)
tVe find this Forcé firnnger in pro-
portion to their Weight in fmall
Magnets than in larger ones J for the
Particles of fmall Magnets, bccatife
they are nearer one another, can the
more eafily tmite their Forces togcthe.r,
Wherefore it is reafonable to expec?,
that the Rays of Light, fince they
are the (mallefi of all Bodies [that we
kuow of) fhoítld he found to have the
firongeji attraclive Forcé of all, How
{iroug this Forcé is, may be gathered
from the following Rule, The xAt-
traSlion of a Ray oj Light, in pro-
portion to the §luantÍTy of Matter it
containSi is to the Gravity which any
projeíled Body has, in proportion to
the ^iantity of Matter containtd in
it, m a Ratio compounded of the
Felocity of the Ray of Light, to the
Velocity of the projeéled %ody, and of
the Bending or Curvatitre of the Line
which the deferibes (n the Place
of RefraSlion, tñ the Bending or Ctsr-
vatnre of the Line which the projeél-
ed Body deferibes ; viz. if the Incli-
nation ofthe Ray to the refraéling Su-
perficies.

N

202

/

RO H AULT's Partí,

Forra of a Tranfpareut Body confifts i i?i the Streightnefs
of its Poyes, or racher, that they crofs each other all’waji
•without any Interruption, and on the other handj a Body
is opake, becaufe mae of its Poyes aye flyeight, oy if thq
be, they aye ?iot peaetyable quite thyough, and all luays.

ferfeUs^ he the fame as that of the pro.
jeéled "Body tothe Horix.on, Andfrom
this Propartion I colleSy that the At~
traólion of the Rays of Light is more
than looooooooooooooo times
greater than the Gravity of Bodies on
the Snperjicics of the Earth^ in pro-
Portton to the ^antlty of Aíatier cow-
tained i» tbem i viz. if Light itikcs
ap abofít fevcn or eight Minutes m
Corning f rom the Sun to the Earth.—-
Now, as ¡n Algebra^ xohere afirma,
the ^antities vanijh and ce afe i there
Negativo ones begin j Jo in Mecha-
TiickSy rohere Attraéiion ceafes^ there
arepufve Vtrtne ought to fucceed . —
Theretore a Ray^ asjhon as it is (¡oaken
effrom a Jhining Bodyyby thevibrating
Ñlotioh of the Parts of the Bodyt and
gets beyond the Reach of AttraÜioni
is driven away xoith exceeding great
Vehcity. Opticks pag, 370.

I, tn the Jlreightnefs of its Pores)
Thus Arijiotle clearly exprefl'eshim-
íelf. The Sight witl not penétrate
folid Bodles, becaufe it can go only
throiigh a Jireight Paffage (this the
Rays of Sun are an Eviáence of and
alfo oitr mt feeing any Objeíis bnt
‘cohat are right bejore its j [when there-
fore the dire¿l Progrefs of the Sight is
hindrcd by the Pores mt bdng all
fireight^ it cannotpaj's through. Bnt
the Sight xoill pajs through jlnid Bo-
d:cs, becaufe the Pares are jmall and
Jireight y fothat it is not hindrcdfrom
gomg through them- Vf^herefore Glafs
is tranfpartnt thongh it be very tUck >
hni a piece of Jd'ood is not tranjpa-
renti though it be very thins becaufe
the Pares of the former are regular^
and thofe of the latter irregular. Ñor
dees their keing largejigrtijy any thing
if they be not (ireight j neither are
rarer Bodics the more tranfparent^
Ttnlefs their Pares are fo difpofed as
ioadmit of a Paffage^Vxoh, 61. Seíl.
II. And indeed that flreight Pores,
or racher fuch as crofs one anocher e-
very v/ay from allSidesj are neceflary
10 a Bíjdy's being cranfparenc cannot be
doubred : Duc how it can be, that noc
only Glafs andDiamonds, but alio
W iier, whofe Parts a.re íó eafy co be

16. 1

moved íhould have íes Pores ílreight,
and eafy to pafs through from all
Sidesj andallWays, and yet attha
fame time, the thinneft Paper or even
Leaf-Gold, for Wííwf of fuch Porei¡
fliould exelude the Rays of Lighc;
is not eaíy to be conceivM. Where*
fore we muft íeek for another Caufe
of Opakenefs,

^ Wc muíl know then, that allBo.
d'es whatfjever, bavein tliem niuch
féwer Parts, and miuch more Pores
or void Spaces, than ís requifite for
the greateft Number of Rays Ai
Light to find a free and qpen m-
íage in flreight Lines all ways wrtL
out running upon the París, For
fince Water is nineteen liff es ligheer,
thácisi rarer than Goldí.*''and Gold
ic felf ís fo rare, that it will ver^
eafijy, without making any Refid-
anee, fuíferthe Magnetick Effluviaio
pafs through ic, and will eafily ad-
mic Q^rkfilver imo its Pores, and
wijl alTonec Water go throughic, that
is, íchas more Pores than folid Parts;
confequentiy Water will have above
forty rimes as many Pores as folid
Parts. 'And indeed you may think,Gpld
and W arer> and all other Bodies (with
great Probability') as much rarer ílill
as you pleaíe. For if roe conceive tht
l articles of Bodies to be fo difpofd
amor.gjl themfelves, that the JnttT-
valsi or tmpty Spaces between thevt)
may he etjua'l in Magnitude ío tbm
all'- and that thefe Particles tnayht
compofed of other Particles much ftnat-
lety xohich have as much empty Spati
betvoecn them> ^as etjuals all the Magy
tiiiudes of thefe fmaller Partkks\
,And that in likemanner^ thefe fmal-
ler particles are again compofed cj
others much fmaller ; all vohi'ch togu
thery are cqttal to all the Pores cr
empty Spaces between them^ and f
on perpctually^ til! you come to jolid
Particles, fuch as have no Peres 01
empty Spaces within them. And if
in any grofs Body there be, for in-
' Jiance, three fuch degrees of Parride!}
the leaji of xohich are folid', this ‘Bd‘
dy xoill have feven times more Veres
than folid Parts, Bnt if there k
• fasr

Chap. 27 - o/ N ATURAL Philosophy. 205

lí. I doubt noE but thaC this Opinioa will be efteem- i6- ^ Con-
ed 3 Conjeflure only. But if ic íball afcerwards be made ' '

appear to have in ic all che Marks of Truch, and that aunT^‘'
all che Properties of Ligbc can be deduced from it ; I
hope that That which at firft looks like a Conjeóture
will be then received for a very certain and manifeft
Truch.

17. And firft, that we are fitted by Nature to per- j^, ae
ceive what we cali Light, though there were nothing are fitted to ,
that bore any Refemblance to it without us, we have ^7;™
a very convincing Experience : For if, when it is the '
Darkeft that can be, we rub our Eyes in one particu-
lar manner, or if by chance we receive a very hard
Biow upon them, fo that the internal Parts of the Eyes
are very much Ihaken by the Blow, we fee Light, and '
very bright Sparks, which ceafe as foon as the Motion
ceafes.

f^fuc^ degrses of Vanhles» the
0/ which are folidi the “Bady
mil have ñfceen times more Pares
than folid^oirtSo If there be five
Degrecsj wi/l have one and

thirty times more Pores than Parts.
If fix Degrees^ the Body will have
Sixty and three timeswor^ Pores tha?t
folid PartSi and fo on ^er^ertnally,
Newc. Opt. p. 243. .v*v
The Keamn therefore Tome
Bodies are Opake, is noc che want of
Pores which are paíl'able on every
Side in ílreighc Lines i but either
the unequal Dcnfity of the Parts, or
the Largenefs of che Poresj either
hlled váth ocher forc of Macter, or
elfe empcy ; by which means the
Rays of Hghc in paíTing through,
are perpetually benc backw^ard and
forward by innumerable Refleftions
and Refraílions, till ac laít they hit
lipón the Parts themfelvesof che Eo-
íly {See the Notes below o?t Art. 35'.)
and ío are wholly excinguiíhed and
loít. Henee it is, that Cork, Paper,
Wood, é-c, are Opake j and Glaís,
Diamonds, .&c. cranfparenc. For
in the Confines of Parts that are
alike, and of equal Denfityj as che
ParcsofGlafs, Water, and Diamonds
are, by reafon of the equal Accra£ti-
ori on all Sides, there is no Reflexi-
ón or Refraéfion j and therefore che
Rays of Light which encer che fifíl
Superficies of chefe Bodies eafily go
on (except fuch as chance co fall
upon the folid Paus, and- are extin-

guifhed. Sec the Notes on Are. gy,
below) in a righe Llne through the
whole Body. But in the Confines of
' Parts which are very unequal in Den-
fity, fuch as the Parts of Wood or
Paper, eompared with each other, or
wich the Air, or empty Space in
che larger Pores of them, the grear-
eft Reflexions or Réfracfions are
made, becaufe of the unequal Ac-
tia6tionj therefore the Rays can by
no means paL through fuch Bodies i
but are perpetually bent backward
and forward, and ac laft lolf. That
this DifcotJthinityof Parts is rhe pr/V;-
cipal Caitfe of the Opacity oj 'Bodies^
will appear by confíderiug that 0 -
pake Subjlances becornc tranfparent^
by fillingup their Pares with any Suh-
fiance of eqrtaly or almoji eqrtal Den^
Jííy with their Parts, Thus Paper
dipp’d in JVater or 0 //, the Ociilus
Mundi btone jiceped in IVater^ Lin-'
nen-Cloth oilcd or varnijhedi and
many other Subjlances foabed in fuch
Liqmrs as will intimately pervadt
the hule Porest kscome by that means
more tranfparent than otherwife 5 fo\
on the contrary^ the mojí tranfparent
Subjlances may by cvacnating their
Pares, or feparating their Parts he
rendered fufficiently opake, as Salts
or wet Paper, or the Oculus Mundi
Stone, by heing dried, Horn by b/ing
feraped, Glafs by heing redneed ío Pon?-
der^ or otherwife fiuwed v-and Water
by beingforme'd into many fmall ‘Btib^
b/es—become Opake. Newc. Opc.p.za^*

iS-Fur-

t-

204 ROHJUL'T’s System Partí.

i8. That i8. Further, That there is Cucha Thing as fubtil Mat-
thereis jacha which peuctrates thc Pores of tranfparenc Bodies, the

Difpoficioa of which to recede from the Center of the
masfuved luminous Body in ftreight Lines, may here be called
fecondccry or derived Lighc, has been fufficiently proved
before, whenwe íbewed the Neceffity of the fecond Ele-
cteni:; and wemay venture to affirm, that noneof thofs
Things would come to paCs without it, which we have
before obferved to come topafs, when weexplainedthofe
Motions which are ufaally afcribed to the 'Bear of a

Vacuum.

I That la- ^ 9' No^hing further remains, but to Chow that lumi-
romo/í/so- nous Bodies do adfually pufh this Matter every Way;
¿iespajh chis ^hich they will be foundtodo, if it betrue, that the Parts
’Wajs^llL arevery fmall, and very much agitated. Let us then ex-
whaticis amine all the luminous Bodies that we know, and fee if
that Fíame gf \vhich thcy ate compofed, be not as fmalL and

emjij í m. jjmch agitated as»we fuppofe. And to begin with Flatnt.

It has been already fo plainly demonftrated, that itjjl
compofed of Parts very ímall, and which raove with
the greateft Celerity, that it is fuperfluous to fay ^y more
about it. * '

50. Whenee 2.0. VVe fee alfo, that there arifes very bright Sparks up-
it is that on ftriking a Flmt againíl Steel, or iwo Fl'mts againfl: each
or an Indiafi Cañe againíl otie, or by

7 rrJhl'ig'‘^ ílrokeing the Back ofa Cat in the Mrk, when theWea-
íOT hará ‘Ba- j, and cold, ' and in a Multitude of other Things.
^eHh^ocheu The Caufe of all which, is only this, that Come of the
Partióles of thefe Bodies being entahgled between others,
when they are ílruck, acquire in flying off, a Motion like
that of Fíame, by which they i n like manner pufh forward
the fmall Globules of the fecond Element.
íuTheCanfe 21. There is fome fort of rotten Wood, and of Fiflies,
o / thtíhiums hgijt to he comipted, which fliine very bright.

»w?Tnio/Now a Body cannoí putrify or be corrupted, but by the
fome 'Fifí-as Motion of its Patts, fome of which fiy off (as is evident
that are car - rottcn Wood, from the Largenefs of its Pores, and
rttm . its Lightnefs, which tender it different from what

it was before,- as a Goal, and the Wood out of which it
33 made diíFer from each other.j We muíl pwn therefore,

T, in a Multitude of other
Things) Thus likewife rab-

br-d very hard in the Dark i ^tick^
Jiher íhaken in a Vacuum \ and a Ghjs
our ot which the Air is exhaufted-j
if >t be turned round very q^uick and

rubbed, willfhinebrighCj notbyim-
pelling or preífing upon the Partir
clesoi the íécond Element, for there
is no fuch Thing s but by fending
fovch ímall Parcicles 'which are the
very Lighc it feif,

that

20 S

^Í-Of thc
Light of
Glüw-mrm,

13 . Ofiie'

Chap. 27. of Natural Philosophy.

tbat the Motion of the Parts which we fuppofe in lumi-
nüus Bodies, • is to be found here alio.

12. It is not fo eafy to tell certainly, what fort of
Motion that is, which makes fome Woms and Flies to
íhine in the Dark : However it is very probable, that
fome fort of Matter is exhalad out of thefe Iníeds, like
the Sweat of other Animáis, and that this puíhes the
Matter of the fecond Element 3 and this is confirmed
I from henee, that they ceafé to íhine as foon as they
I are dead.

'23. The S«» and the are the mofl: luminous Bo-
dies of any that we know 3 but by reafon of their great ¿y tht
Diñance, it is impoíEble to make appear by any Experi- starT!'^
ments taken near them, that all their Parts are in Moti-
on 3 all that we can affirm, is only this, that we do not
obferve any Thing to the contrary : And íin« they pro-
duce the fame Effedfs in us, that Flame^does, we
ojght to think, that they refemble it in that by which
tMe EfFedls are produced, toz. in the Motion of their
Parts.

24. f^t were true, what they fay of a Carhuncle and íe^.rhat m-
a Diamond, viz.. that they Ihine in the Dark ; I íliould m
freely own, that I am miftaken in all that I have faid Jha 7 ‘tíéfe-
about Light 3 for there is no Probability, that Bodies fo lateofaCar-
hard, ílrould be com-’pfed of Parts which feparately are 7 ‘DUimfd,
in any Sort of Agitación. But it is certain, that thefe

are only idle Stories, told without any Proof, and receiv-
cd by credulous Perfons, for I have often times experien-
ced the contrary my felf.

25. Tistrue indeed, that a Diamond Ihines very bright aj'.

in a darkiíh Place 3 but the Reafon of this is, becaufe it Sri^ht-
is fo cut, that the Sides refledt all the Light which they
receive towards the fame Part, as fhall be more fully fítsin.
explained * afterwards, when we come to treat of the * 46-

Refradlion of Light.

2(í. We havelately had an Account from England, that
‘ fome Diamonds rubbed in the Dark, have íhined fo »/ «
bright for a fliort time, tbat a Word or two might be
read by Light of them. I have not obferved chis in any mbkcá.

!• // to be found here alfa.') The.
famous Mr. "Boyle niade an Experi-
ment of chis Matter, which is very
well worth caking notice of. He
put a piece of roteen Wood into the
Air Pump, which wasin a Manner ex-
úngui/hed and ceafed to íhincj when
ihe Air was exhaufted : but npon let-

ting the Air in again, irfeerned to be
newHghted, and íhined as before. See
the Philofuphical Tranfafl* Numh. ^ 1 .
For this w'as true Fíame, and like
all other Fíame cannot be preferved
without Air. '

2. Some Dlumonds rubbed in the
Dark) See Are. 20, above.' - •

Diamonds

2o6 RO H a ULT*s System Partí,

17. Of the
Boulogn-
Stonc.

2S. The
Reafnn of
this Stone's
fhining.

19» kA Con"
jirmation
hereof

Diamonds that I have tried ^ however it tnay be truc,
withouc contradidting any Thing that I have hitherto
wrote. For the Rubbing may raife fome Agitationj ¡i
not in the Parts of the Diamond, yet at leafl: in fonii
Matter contained in the Pores of it, which continuing in
Mocion in the fame mánner as the Fíame in the Pores o!
a burning Goal, may for fome time pufh the fecond Eb
ment which is all round it^ and difpofe it to raiíe a fmal
Senfation of Light.

27. Though we have no Jewels which íhine in th
Dark, yet we have a Stone that is truly luminous ; Th»
Stone was accidentally found by an Italian ChjmiJl neai
Boulogn in a hollow Place cauled by a Torrent. Afcei
having put it inco a Fire for íix Hours, be took it out,
and let it coolj andwhenithad been expofed to the Ligbi
of the Air^or fome time, upon carrying it afterwards in-,
to the Dai'k, he firíl perceived it to look like a Fite-
Goal covered over with a few Afhes. I have feen f¿t
fhine near half a quarter of an Hour, after which m
Light vaniíhed, but by expofing them to the Light of the
Air for a íhorttime, wecould malee them íhine í^'inwhen
we pleafed.

28. The Reafon hereof very probably is, that the Fire
has made this Stone extremely porous, fo that among the
Parts which are almofl; wholly disjCíned from each otheti
there may be fome i ío eaiy to be put in Motion, tha:
the Light of the Air alone is capable of agitating them;
and they may be fo difpofed to retain this Motion, thaf
they may keep it after they are removed from amongíl
the luminous Bodies, which put them in Motion j and
this is confirmed from henee, that whan this Experiment
is often repeated, tltefe Parts exhale, and the Stone quite
lofes its fliining Quality j which Quality cannot be prefer-
ved above four or five Years, though the Stone be
carefully fhut up in a Box, where no Light can come
at it.

2p. For a further Gonfirmation of what. has beefi fai^i
we may obferve, that if this Stone be kept too long in
the Fire, or though it be kept in it but fix Hours, yet

I. So eafy to he put in Motion)
In much che íame manner may che
FhofphoTtts be accounred. for , (che
manner of preparing it, is at large
explalned by the famous Mr. Boyíe,
to whom i refer you,) for ic i.?
very probable, that lomeíulphureous

Parts of the Uriñe» prepared pver a
veryhotFire, arefo voladle and eaíy
tobe put in Motion, that they are
turned into a k'md of Fíame, by the
Agicadon of che groITer, or perhay?
oírbe finer Air. .

íf

Chap. 27 * o/ N atural Philosophy. - 207

if the Fire be very hot, all the Parts of it which cannoC
refifl: the Fire, may be carried ofF, and then the remain-
ing Parts may be fo heavy> as not to be íhaked by the
Light j in which cafe the Stone ought not to Ihine, and
fo we find by Experience.

30. Having thus íliown theTruth of thofe threeThings 30. Tkat
which comprehend the Whole of our Conjeture, about
Ymútive or Origmal J^ight, concerning what they cali
femdary or derivative Light, we obferve firft.3 that be- to alL
caufe it does not coníift in the aflual Motion of the fub-

tle Matter whicíh filis the Pores of tranfparent Bodies, buc
only in the Tendency or Difpofition which this Matter
has to Motion ; it neceffarily follows, that luminous Bo-
dies, be they never fo diñant, ought to propágate their
Forcé, and ‘ to aíFedour Senfesin a Moment of Timej
I becaufe the Matter which is puílied, being extended eve- ,
i ry way without Interruption, like 3 very long Stick 3 the
j Ijminous Body cannot puíh forward the nearefb Part of
I iwbut atthe fame time it muft impelí the furtheft Part
likewife.

31. .%t perhaps fome may think, that this Train of

3 1. -í Vif-

Matter which is extended from one Point of the {ami- ficnity abmt
nous Body, to a Point of the Objedt which it H^umi- 0/
nates, and which is called a Ray of Light, may more pro-
perly be compared Uva Thread than toa Stick, becaufe its
Parts are not ib firmfy connecSed together, as thofe of a
Stick are 3 and fo it may be conceived, that as we can
move one end of a Thread, without moving in the leaíl
the other End, fo the luminous Body may impel the Mat-
ter of the fecond Element to which it is applied, with-
out neceffarily continuing that Impreílion to any great ’
diftance. However, if we confider, that the World is
full of Matter, and that a Ray of Light is always fur-
rounded by a great many others, which hinder it from
bending as a Thread does which is not furrounded
by others, we íliall be of Opinión, that every Ray of

í. Tú ajfe¿l our Senfes in a Mo^
nant) Ir appears now from the Phe-
nomenaof Jupiter’s Satellites, jwhich
get inco the Shadow of Júpiter a
Jíttle fooner chan they ought to do*
when rhe Earcti approaches cowards
Júpiter j and on the other hand,
come outof the Shadow a líttle la-
fer than they ought to do, when
ihs Earth depares from Júpiter (as
many eminent Aftronomers haveob-

Light

lerved; Chat Eight (which is a real-
Body) is not propagated in a Mo-
menc of Time» buc cakes np. abouc
feven M' ñutes in coming from che
Sun to che Earth, whrch is abQuc
yooocooo of Miles (See Metot. Opt.
p» zyi.) What íurpriiing Thingf
follow from Lights not being pro-
pagatedina Moment, but in acertain
Space üf Time, Yüu may íee in the
Notei on P^tr-t il. Ch^ xxv.

20S

ROHAULT^ s System Partí,

32. That a
S<}dy may
propágate its
uííiion thro’
An interme'"
diate Liqnor,

33. That to

Jhottld be con-
taineá. in any

Light ^ oaght to propágate the Forcé of the luminous
Body in the fame manner, as if it were as íliíF as a Stielc.

32. In order to explain what is difEcult in this Matter,
let US compare this Adion of the íecoad Element which
tranfmits Light, to che Adion of Water contained i'n.a
long chick Tube ftopped ac thelower End,- and chen íet
US confider, that all the fraall Threads of which this grofs
Column of Water is corapoíed, do every one in parti-
cular prefs with its whole Weight upon the Bottom j and
that if wé pour in never fo lictle Oil, ic will prefs
upon the Bottom in the fame manner as ¿f we had pour-
ed it upon a ftifF Stick.

33. If this Comparifon does not feem juft, becaufe in

^A^aánkis‘ '^he Water is contained in a Veílél ; take

mt. necejjUry anochet : Suppofe the Surface of the Earth, inftead of
thí íj^mt being unequal and rough as it is now, were round and

and imagine it to be covered all over with Wa-
ter to a certain Height,- then would every Point of the
Earth’s Surface be preííedupon by the whole Weight^f
the Threid of Water which correípends to ic j now com-
pare the Adion of the Ra/s of Light to che Adion of
this Water, and you will find, that chey are o^p'able of
ading in the fame manner, as if they were as ftiíF as a
Stick.

34. It is true however, and muft begranted, that there
is fome Difíerence between thefe líl'to Things ; For the
Threads of the Water approachnearer and nearer to each
other, and tend to the fame Center, whereas the Rays of ,
Light go from che Center and fpread themfelves towards
the fpherical Superficies which we may conceive al!
round them : Buc this DifFerence will only be of ufe, to
íhow US the reafon of a very remarkable Property of
Light; which is, that the Impreffion of the luminous Bo-
dy does not come entire to the Objed; but is weaken-'
ed and diminifh’d a lictle, according as ic fpreadsit felf, and
proportionably to its Diftance from the Center of Adion,
In order to explain this, lee os fuppofe the Tube ABC,
which grows wider towards the Top, to be filled with
Water as high as DE, and that afterwards with a Sy-
ringe we put as much Water in at che End A of this
Tube, as will fill the Space AFG, which is of a conlide-
rable Height, but of a fmall Breadth. Ic is certain, that
this Addition of Water, will raife up the Water at FII a

34. TVhy the
2 d¿iion of
Light grows
meakcTi the
more dijiant
the Xumiuons
’Body is.

Tab. IV.
Fig, 3.

T. Oítght to propágate the Forcé)
To propágate íc incleed, but not
íq ílraiglu Lines? as Light is really

propagated. See the Notes on Art. 15’.
abovcm

209

Chap. 2/. of Natural Philosophy-.

little, but that it will be fcarce fenfibly raifed at DE. Now
this explains the Nature of Light perfeótly well. For as
we cannoc fay tbat the Water at DE is noc raifed at al!,
but only that it is raifed but a very little ; So we may
conclude, that the further the Rays of Light are diñanc
from the luroinous Body, the weaker they are j which a-
grees with Experience.

3y. Now as we are certain, that a Body in Motion al-
ters its Determination when it tneets with another Body
that reflíls it ; So likewife we may conclude, that Light,
when it falls ' upon the Surface of a folid Body ought to be
turned back or refleded. Thus for Example, if the fmall
Globales which are in the Line CD reprefent the Parts
of the fecond Element compofing a Ray of Light, which
falls upon the folid Body AB, its Adion ought to be con-
tinued towards E, along the Line DE, in fuch a manner,
as that the Angle of Reflexioji BDE ought to be equaí
to the ofincidence ÁDC, that is, this Adion ought to
In propagated in the fame Lines that the GlSbule Cwould
íKfcribe, if it were alone, and moved in the Line CD :

For

%

I.. Üpon the Surface of afnlid^Body)
The Reflexión of che Rays of Líghc
is caufed, not by falling upon the
Parts themPelves of the^fleñing
Bodyj but by a cefcaln PoV^r eaual-
ly diffufed atl over che Surface of the
Body, whereby it a6i:s upon the Ray
to atiraft or repel Le, voUhout imme-
diate Contañ ’t by which fame Power
in otherCircumífances the Ray is re-
fra£ledi andby which fame Power
it is at firft fenc forth from che lu-
cid Body i .as che fore-ciced admira-
ble Períonhasdemonftrated by many
Arguments.

I. Thüugh thofe Gíaffes which we
cali plain and poliíhed, do indeed
appear to the Eyeto havea fmóoch
uniform Surface i yec in reality, (íince
poliíhing is noíhing elfe but wear-
ing away and breaking che Procu-
berances of che Glafs, with Sand,
Putey, or Tripoly) their Surfaces are
Very far from belng plain and fmooth ;
Now if the Rays of Light were re-
flected by impinging on the folid
Parts of che Glafs, their Reflexions
could noc be ib exa¿t and regular,
as we find they are j nay^ the Rays
ought to be difperled all Ways, al-
moft as much by the beñ poliíhed
Glafs, as by che rougheíie Sce
Opu p, 2.40,

II. If the red and blue Rays

which are feparaced by a Prifra [th&
manner of doing vohich» See in the
Notes on Art. 6 y. belovo.) be ail of
chem caft on a íecond Prifln, ín fúch
manner, that they are all alike inci-
denc upon its the fecond Prifm may
be fo incUned to the inciden: Rays;
that thofe which are ofablue Colour,
íhall be all refleSled by it, and yec
thofe ofa red Colour (chough falling
with 'the íame Obliquity) precty co-
plouny tranfmitLed. Nowif the Re-
flexión be cauíéd by che impinging
of che Rays upon che Parts of che
Glaísi how comes it 10 pafs? that
w'hen all the Rays tall with che fame
Obliquity, che Biue íhould wholly
impinge on the folid Parts, fo as to
beallrefleffed, and yec che red find
Pores enough in che fame Place to
be in a greac meafure cranfmitced ?
Pag. 239. .

III. Where cwoGlafles touchone
another, there is no fenbble Reflexi-
ón, and yec there is no Reafon why
the Rays íhould noc impinge on che
Parts of Glafs as much when con-
ciguous to other Glafs, as when con-
ciguous to Air. Ibid.

IV. When the Top of a Water-
bubble, madebyebe workingup of
§oap and Water;, by che concinual

P íub-

5 y. Why
híght meet^
ing with cer-'
tain "Bodies
ought to be
Tfjiecied-.

Tab. IV,
Fig. 4.

210

1

R.0 H AU LT^s System Part I,

For it is evident, that rhe Globule D ought to have a
Tendency, and to be diípoí'ed togo where it would really
gOj if its Po'wer were fut into aéi. And fince this Glo-
bulcj upon meeting wich the Body AB, would neither go
towards G, ñor towards FF but only rowards F j it muft
be allowedj chat it is the Globule F only which is impel-

led

fubfidlng and eüíhaling of the Water
growsvery ihini there isnoinani-
Ícít Reflexión, noc onJy ac che leafl
Tlilclvnefl'e.',, but alfo at many othcr
Thickneflca of the Bubble, which are
conrinually greater and greater ; and
yecin theSuperficiesofchechlnncd Bo-
dy, where kisof any one Thicknefs
there are as many íolid Parts for rhe
Rays to impingeon as where it is
of any ocher Thickners. Ib¡d.

V. If the red and blue Rays fe-
parated by a Prifra.(íA{? manntr of
iiohig rohích, as toas faid heforíj ynn
may fue m the Notei 07 i Art. óy. be^
loto) be afcerward caft diftindlrly and
íiicceííively upon a thin Place of
any tranfparenc Maiter, whofe Thick-
nefles grow continually greater and
greater (fuch as a Place of Air con-
tained becween a plain Glaís, and a
Clafs chut is a liccie gibbous, fuch as
ihe Objedt-Glafs of a long Teleí^
cope) this Place in che very fanio
Part of it will refleóf all rhe Rays
that are of one Colour, and tranfmic
alUhon: chat areof anocher Colour 5
in/¿/ffr(?wíPaftsotic, itwill cranrmíc
Rays of the fame Colour ac one
Thicknefs, and refledt chem ac ano-
ther, and chis by innumerable Fices.
3SI0W ic is noc any way 10 be ima
giced or conceivedí that ic can í*o
happen by chance, that in che very
fame Pare of the Place» and wich che
very fame Obliquity of ' che Rays,
all che Rays that are of one Colour
ílrould impirge upon the folid Parts,
and all che Rays that are of anocher
Colour íliould hit upon the Pores
only and chat in dijferent Pares of
the Place, in one Place the blue
Rays íliould all impinge upon the
Parts of che Body» and the red Rays
nin all into the Pores; and in ano-
ther Place where the Place is a lítele
thicker or a liccie thlnner, on the
concrary the blue Rays only íhould
run all into the Pores, and all che'
xed Rays impinga upon che Parts,

240.

VI, In the Paíliige of Lighc ouc

.OÍ Glafs into the Air there is a.Reflexi- ’

on as ñrong as in íes Paflage out
of Air inco Glafs, or raiher a Ucele
flronger, andby many degrecs ftron-
►ger iban in its Paflage ouc of Glafs
into Water. And ic feems not pro-
bable» Chat- Air Ihould have morere«
fledling Parts than Water or Glafs,
But if that íliould poíflbly be fup-
pofedj yet it will avail nothing, for
che Reflexión is as fírong or flronger
when all che Air is removed from
the furcher Surfacc of the Glaís, as
whenit isadjacenc to it.p. 237. Now
if any one íhould imagine accoráV
ing to the Opinión of Cartesj MI:

■ che íubcle Matcer ac The furcher Sur-
face of the Glafs is denfer iban any
other Matcer v/hacíüever| 5 hid upon
that Account more flrong to reflefl:
Lnghc than any othcr Budies , be-
fides that we have before demon-
flraced that chat Macter isonlyafifti-
c'ous Tlihig ; and that if we íhould
allow cSíTMatter, and its Power co
refltíct Light, che Light could aotbe
propagaced by it ac the Beginning,
but muft immediacely be all refleéled
back upon rhe lucid Body as fooii
as ic is fenc forth from it ; belides
thefe 1 íay, he will be cunvinced of
che Falfity of this Fid'tion by che
füllov/mg Experimenc.

VII. If Light in its’PalVage ouc
of Glafs into Air be incidenc more
; obliquely chanat an* Angla of 40 or
41 Degrees, ic is wholly refleéled,
if lefs obliquely, ic isin greac mea-
fure tranfmitted. Now ic is not to
be imagined, chac Light, ac one De-
gree of Obliquity, íhould meec with
Pores enough in the Air to traní-
mic the greater Pare of it j and at
aqother degree of Obliquity íhould
rneer wich nothing but Parts co re-
fledfc ic Y^holly ; efpecialiy, confider-
ing, chai in its paflage oucof Air
inco Glaís, how^bliquefoeverics In-
cídence be, ic finds Pores enough in
che Glafs, co tranímit a greac Pafc of
ic. If any Man fuppofe, chac ic is
not refledted by the Air, bueby che
oucmoñ fuperficial Parts of che , Glafs»
chac will appear to be falfe, by ap-
plyin»

■*!0

2II 1

Cbap. 2 ^i of Natural Philosophy.

led by íc, and which receives ks Adion. And this is
coníirmed by Experience. For when the Lighc falls up-
on the Surface of any Opake and folid Body, as Gold or
Steel, we fee its Rays are refleded, and the Angle of
this ReJIexmi is equal to the Angle of hicide7ice.

3 (). Now this being fo in one folid Body, fuchasGold ^6.Thattk!-e
br any other Metal ^ as it is a general Truth, it ought io
extend to all Sorts of folid Bodies, and the Light ought bat repa
to be refleded in Angles equal to thofe of their Incidence.
Wherefore lince the Pores of twotranfparent Bodies which
touch each other, cannot exadly anfwer to one another ;

plyjng Water or Oil behjnd fome
pare of the Glafs inftead of Air,
Forfo ina convenienc Obliquity of
the Rays fuppofe of 45 or 46 De-
grees, at which they are all refiefted
where the Air is adjacent to the
Glafs» they wiil be in greac meafure
tranfmitted where che Water is ad-
ecene toit; which arguess that their
ileflexion or Tranímiflion depends
on che Conñicuüon of che Air and
Wat-a^orOil behind the Glafs, and
not oifthe ñriking of the Rays up-
on che Parts of the Glafs, viz, that
the Rays are not refledled cill they
gec to the furcher Surface of che
Glals, and begin to go ouc of ic :
For if when they are '^p^ng ont of
it, they fall upon Oil or Water,
they go on, becaufe che Attraítion
of the Glafs is almoft ballanced and
rendred ineffeílual, by che concrary
Aterañion of the Liquor that ñicks
to ic. But if the Rays, which go ouc
of che furcher Supertícies, go into a
Vdcwwii which has no actraftive
Forcé, or inco Air which has very
licile, and cherefore cannot ballance
the Attraffion of che Glafs, and ten-
der ?c inefFeftual 5 chen cheartraílion
of the Glafs refleffs chem, by draw-
ing and bringing them back. And
thisisftill more evident, by laying
logether cwo Priímsof Glafs, ortwo
Objeft-Glafles of very long Telef-
copes, the one plain, che other a
litcle convex, and fo compreíling
them, that they do not fnlly touch, nnr
are too far afiinder : For che Light
which falls lipón the farcher Surhee
of the firíl Glafs, where the Inter-
val becween che Glafl'es, is not above
the Ten hundred thouíandth pare of
an Indi, will go chrough that Sur-
face, and chrough the Air or Vacunm
becween the Glaílés, and enter into
the íecond Glafs. But if the fecond

p

Glaís be calcen away, the Lighc
which goes ouc of che fecond Sur-
face of che firft Glals inco rhe Air
or Facunm that is between the Glaf-
fes, will not go on forwards, buc
curns back inco the firft Glaís, and
is refleéled. From whence it is
evident, that the Rays are dr'awn
back by the Power of che firíf
Glaís, there being nothing elfe
to tiirn them back. p. 238, and
347. And henee ic is alfo raanifeft,
as was before oblerved, that the.
Rays are not refleíled by any fub-
tle Matear or jocher, becauíe that
Maccer ought to refieá chem not at
all che lefs, when che fecond Glaís
is fo placed as not quice to touch che
firft, than when ic is quice caken
away.

hajlly% If any one íhould ask ;
becauíe wehave aferibed the Reflexi-
ón of Rays to the A^ion of the
whole Superficies of Bodies, wich-
ouc immediacely touching chem i
how ic comes to pafs, that all Rays
are not refledfed by all Superficies 5
buc whüe fome are refleéled, ochers
are refrafted and enter in : This ex*
cellenc Perfon fiiows, thac there are
cercain Vibradons (or íome fuch
klnd of Properey) both in the Bo-
dies themfc'lves, and inthe Rays cf
I.ighc, impreffed upon the Ráys,
eicher by the Añion of the Body
which emics them, or by che A£li-
on of fome other Bodies ; whence
it comes to pafs, that thofe Rays,
which are in thac Pare of chtir
vibración which confpires with rhe
Modoc of the Parts of the Body,
enter inco the Body, and are cranl-
mitced by RefraStionj and thofe
which are on the other Pare of their
Vibración, are refleíled. ScsNewf»

and

212 RO H AUL'T’ s System Partí,

and therefore many of the Pores, of Air for inftance, may
meet wich the folid Parts of Water, Glafs, or Chryílal ; ic
is impoffible, but that tranfparent Bodies muíl refledl
fotne part of che Light which falls upon their Surface ,• and
they muft refleót fo rnuch the more, as the Rays fall
more oblique, bécaufe in that Poficion they meet with
more of the Iblid Parts of the tranfparent Body upon which
they fall.

37. Hoa the 37. Let usnow confider, what will happen to Rays that

Raysefa^ht pjfg q^jj; of ohc traníparcnc Médium inco another, upon
"Is whofe Surface they fall obliquely. We forefee i that they

caí of one ought to be rejraíícd agreeably to what was faid before
Med'íumXío coHceming Refraclion, becaufe thefe tranfparent Bodies
mothex. beiiig of a different Nature, the one may afford an eaíier

PaíTage to the Light than the other, and fo the Rays óught
to be leís inclined, or nearer to the Perpendicular on that
Side which more eaGly admics them.

38. Thehar^ 38. Nor are we to think, that a tranfparent Body will
der a tranf- afFord ío much the eañer Paffage to Light, by how muc^

eaíier it yields to other groíTer Bodies which make
exifier mil the Way for themfelvcs, by removing its Parts : the

thnlhf/ contrary : For as the PaíTages for Light are already made,

1. That they ought tohe refraíled)
The Rays are refraftecl, noc by fai-
líng upon the very Superficies ot’ Bo-
cliesj but wicliout irnmedíace con-
tare, but that very .fame Power by
which they are emitced orrefledledj
exercing ic felf diííerently in diffe-
rent Circumñances j as may be de-
moHÍlraced by che fame Arguments
as were before made ufe of abone
Re£le£fion withoucConca6t j andalfo
by che folio wingAines.

I. ‘Bccaufe rohen Light goesoutof
Glafs inta Air^ as obliquely as it
can pojfibly doy if its Incidente be
inade fill more oblicjuej it becomes
totally refelled. For the Poveer of
the Glafs^ after ií has refra&ed tbe
Light as obliquely as is poffible^ if
the Incidente be .made fill more
oblique, bccomes too firong to let
any of its Rays go throngh^ and
by confequcnce caufes total RefeX'
ions,

i. Hccaufe Light is alternatcly
refielied and tranfmitted by thin
lUates of Glafs for many Succejf-
ons accordingly as the Thicknefs oj
the Píate increafes in an arith^
metical Progrejfion. For here the i
Thicknefs of the Glafs determines^

vohether that Poxoer by which Glafs
alis upon Light floall caufe H to be
refeüed, CV fnfer it to be tranf-
mitted,

3. becaufe thefe Surfaces of tranf-
parent Hodies which have thegreat-
efrefraéling Power, refieli the great'‘
efi quantity of Light, New:. Opt.
p. 244.

4. Becaufe, although theForcesof
Bodies ÍO refleíland refracc Light, are
very nearly proportional to che Den-
fities ot che fame Bodies s yec
unftuous and fulphureous Bodies
refrafl: more than others of che
fame Denfity. For the Rays aéf
wich greater Forcé upon thofe Bo-
dies co fec them on Fire, than they
do upon others; and thefe Bodies
aót upon rhe Rays again with
greater Forcé by mutual Ateraftion
to refrañ them. p. 24,y, (¿^c.

Lafly. Becaufe, not only the Rays
which are tranfmitted chrough Glafc
are refle£led ; bu: alío thofe which
are near che Excremities of ic in
Air or in a Vacuumy or even thoíe
which are near the extreme Parts
of any opake Bodies (^as the Edges
of Knives, ó-c.) are benc by che Ac-
craófion of the Body. p, 253. &c,

ic

2.13

Chsip. zy . of Natural Philosophy.

ir can move fo much the eafier as the Parts of the Bo-
dy through which it paffes, are more difficult to be puc
out of their Places ,• becaufe ir is the iefs Hable to lofe
its Motion in paffing j in the fame manner as a Bowl
will mn eafier upon the firm hard Ground, than upon
foft Groundj or upon the Graís. And thus as Water
is in fome Senfe harder than Air, and Glafs harder than
Water, and Chryftal harder than Glafs, it follovvs, ' thac
Light ought to pafs more eafily through Water, Glafs
and Chryftal, than through Air; and its Rays ought to
be lefs inclined, or to approach nearer to the Perpendi-
cular in thefe Bodies than in Air.

39. This may be tried many Ways ; I will íliow you 39.
one that feems to me very evident. I caufed a
Box ABCD to be made, with a Cover to it of the
fame Metal. The Bottom BC was a Piece of Fenice mt óf

Chryftal, under which I glued a piece of Paper, w'ith
feveral Marks made upon itacPleafure. I expofed this Box Xab. iv,
%p the Rays of the Sun, that a Ray, fuch as FE might F‘g- í-
pafs through the Cover at the Hole E, and looking un-
derni^h, I obferved the Point G, which the Ray carne
to; tnen withouc altering the Situation of the Box, which
was full of Air only, I filled it with Water, which I
poured in at the Hole M ; then í obferved, that the
Ray did not come fo far as G, but only to L, fo
that it was nearer the Ferpe 7 ídim¡ar HI, than it was be- '
fore.

40. Now to find whether a Ray paffing oyt of Water
■ into Air be turned from the 'Perpendicular, W'e may make yerimmt oi
ufe of a very common Experiment. We may put any Refrad!-
Body, a piece of Money fuppofe, at the Bottom of a
hollow Veffel, which conrains nothing but Air ; then we /faur int/
may move our Eye B back, till thé Edge of the Vel-

Tab. tv.

fel juft hides the Objeft A; then let the VeíTel be fil- pig/g.
led with Water : After which, the Objeíl without ha-
ving changed its Place, will begin to appear by the Ray
CB, which Corning from A by C, will be bent, and re-
moved the Perpendicular ECF, whereas otherwife the
Ray would ha ve gone ftreight on to D.

I. Thát Li{[hí QH^ht to pafs more
eafly) Mr. Le Clcre has commicced a
fiirprizing Miftake here. Therefore,
fayshe, the greater the ReJjJiance oj
the "Rody is upon vohich the Ray fallst
fo much the more does it recede
jrom the Perpendicular, and the lefs
^be Reffance) the lefs does it recede*

Wherefre a Ray fall/ng upon Tfa~
ter out of ^ir, gocs jurther from the .
Perpendicular i on the contraryy a
Ray Corning ott' of Water into' Air apm
proaches ncaytr to the Perpendicular >
kecanfe Air reffs i: lefs than Water,
Phyf. ‘Booi’ V. Chap, viii. Seé?* 17,
comrary to all Experience.

3 41. Be-.

\ R0H4ULT’s System Partí,

41 . ofihe. Becaufe Refradtion will be of greac Ufe hereafter,
^UghtfT¡¡iíg worth while to explain the Nature of it fully, by
throagh a ^ confiderÍDg how it is made, when Lighc paíTes out of
^ríh ^iv”' various forcs of Figures. Suppofe

Place * a triangular Prifm ABC, up-
en one Side of which, fuppofe AB the Ray I)E falls
obliquely. From what was faid before concerning the
Rays pafling out of Air into Glafs, it follows, that it
ought not to go on in a ftreight Line to F, but to G,
in order to approach nearer the Line HEI, which is
fuppofed to be dravvn through the Point E upon which
the Ray falls, and to be perpendicular to the Surface
AB. After which, the Ray EG paffing obliquely out
pf Glafs into Air, ought not to go diredtly to L, but
■ to M, becaufe it is turned from the Perpendicular
NGO.

41 . ofthe 42. Suppofe now a Lens or a Glafs convex on both
RefraSimof Sides, fuch as is rcprefentcd by the Figure 2B3K3 and
tSongTa imagine a great many parrallel Rays, fuch as AB, Cl|^
Convex Glafs. EF, to fall upou its Surfacc," now in order to find out
Tab. IV. how thefe P,.ays ought to be refrafted, we m^ firft
F'g" 8. draw through the Points B, D, F, Lines perperwicular
to the Glafs, that is, the Lines ABK, HDI, LFM,
tending towards the Point G, which I fuppofe to be the
Center ofthe Superficies 2B3. T^is being done, we
inay confider, that the Ray AB, b*hg in the Perpendi-
cular it felf, ought not to be at all refraded as it palfes out
of Air into ^laís, but to go ondiredly towards K, where
it falls again perpendicular upon the Superficies of the
Air 2K3 fbecaufe it comes from the Point Pv, which is
the Center of this Superficies) and therefore it will con-
tinué to go Ifrait on ílill towards G, without any Refra-
dion. But as to the other Rays, fuch as CD, and EF,
becaufe they do not fall perpendictilarly, it is evident,
that they will not go diredly to O and N, but will ap-
proach nearer to the Perpendiculars HI, LM, and go to
Q^and P, and by this means they will tend towards the
Ray ABK,- and becaufe, having drawn the Lines TQI,
SPM perpendicular through the Points P and that is,
the Lines wliich tend to the Point R, we find that the
Rays DQ, FP fall obliquely on the Surface of the Air,
we conclude, that they will be refraded, and go from
the Perpendicular. So that DQ_will not go diredly to X
but to G, and FP alfo will not go diredly to V, but tq

I. oí Triangula} Frifm) See the Notes on Art. bclow.

the

Chap. 2^. o/ N atural Philosoph Y. zig

the fame Point G. The fame may be demonftrated of
the Rays; that fall on the other Side of AB, which will be
te.it foj, as to interfedt the firíl ' ibmewhere near the Point
G j thus we íéCj that it w the Froperty of a Convex- Glafs,
to colleíi togetber the Rays of Light tobicb fall parallel
npoti it.

43. If whilft the Glaís remains in the fame Situationj 43-
parallel Rays fall upon it from forae other Place, we fhall

find that they willmeet together in fome other Point, and chicheme
notin G j thus if they come from the right Side of thofe
before drawn, they will meet on the left Side, viz. near ‘

Yj and on the contrary, if they come frorn the íefe
Side, they will meet on the right Side fomewhere near Z.

44. Let US conílder in the Third Place, A Glafs that

is thinner in the Middle than at the Edges, that is, a Glafs u¿hc pajjing
concave on both Sides, fuch as isreprefented by GBHIMK, througha
and fuppofe the parallel Rays, AB, CD, EF, to fall up-
on it. No.w in order to fee how they ought to be refracled, Tab.' iv.
%’US erecl; Perpendiculars at the Points B, D, F, where f'S- 9 -
they enter the Glafs : This being done ; lince the Ray
AB .«lincides with the Perpendicular, it will enter the
Glafs as far as M without any Refradfion, where becaufe
it falls perpendicularly upon the Superficies of the Air, it
will no more be refracted at going out, than it was at
entring into the GlaR, and confequently it will go dired-
ly to L. But became the Ray CD falls obliquely upon
the Surface of the Glafs, it will not go diredly to P, but
will turn to Q, becaufe it tends towards the Perpendicu-
lar NDOj and becaufe the Ray DQ^falls obliquely up-
on the Surface of the Air alio, it will not go direclly tp
T, but will be refraded towards V, becaufe it goes from
the Perpendicular RQ.S. So iikewife if we examine the
Ray EF, we fliall find by the like Way of Reafoning, that
it will go to Y, and from thence to Z- Whence we fee,
that it is the Froperty of a Concave-Glafs ^ to difperfe the Rays
vohiclofall parallel upon it.

I. Samexiohere near the Vaint G)
For :he Rays are not colleñed to-
gether exaílly into the
Tab. IV. fame Flace, and the Fo-
Fjg. 8. cus is not in a Poíntj but
in a fmall Líne, that is,
inpartof ^ne Lineis-G, ib that fome
of che Rays meet with each other
nearer the Point IC than others of
ibem. .Thus for Inílance, if the
Glafs be eqiially gibbous on both
Sidesj that Line will be X of the

whole Thicknefs BIC. See Hn^en's
Diopt. Prop. 27. p. 94. and
rom, «Si-í?. V,

2. To difperfe the Rays) In fiifh
a manner that they may feem to
come from a fmall Line,
or füch Pdi't of che Line Tab. IV#
AB as che íoremention- Fig. 9*
ed fmall Line was, into
which they were gathered in pallin^
through a Convex- Glafs.

2i 6 • ROHJULT-s Syst2u Partí.

45-. Horothe Lct US cotifider iu the Fourch Place, a Glaís cut
fifaedin" feveral Surfaces on the one Side, buc plain on the

faffing lhrt>‘ other, fuch as is reprefented by the Figure ABCDETS,
a Gtafs that fuppofe the Rays FG, HI to fall parallel upun it ;
mlnyf^per- Draw Pcrpendiculars in the Points G and I ; then be-
J¡ci!s. caufe, from what was before faid, thefe Rays ought to go

Tab.^v, towards the Perpendiculars, we are fure that they will

’ bend towards K and and becaufe they again fall ob-
liquely upan the Surface of the Air ST, we conclude that
they will be refraóted a fecond Time ; fo that GK will
tend towards L, and IQ^ towards M ; and becaufe all the
parallel Rays that fall upon the fame plain Superficies, arq
equally inclined to it, they will be equally refraded, and
confequently will be parallel when they come out, fo that
thofe which fall upon the Superficies BC will go along
with the Ray KL, and thofe which fall upon AB,
CD, DE, will go along with the Rays QM, PN,
and RO.

4S. mirem 46. So that if the Surface TS were covered with ^
the Lajire of opake Body which receives all the Rays of Light that fall
^stmTs\on- upon the Superficies AB, BC, CDjDE,- it isevidei^ that
none of them will come upon the Parts SQ^and lÍT, and
confequently they will look darker , whereas the Part QR
receiving all the Light which falls upon every one of the
Surfaces ought to appear very brigk •, and herein con-
fifts the Luftre of a Diamond and oraer precious Stones
which are any way tranfparent. For they' will not fhine,
unlefs they be cut with a great many Superficies in
fuch a Manner as to tura the Rays of Light towards
one Place at the Bottom, where is a fmall Píate of
Gold or Silver to receive the Light, and refled it back
to our Eyes.

47. Of the 47. Lañly, Let us fuppofe a plain Glafs of equal Thicknefs
every where, fuch as ABCD, upon which the parallel
ihVr^am Rays, EF, GH, IL, if they fall obliquely, fall with equal
Glafs. Obliquity, fo that they are equally refraded, by approach-

v\g,'z/ every one of them towards the Perpendicular, and
• ’ therefore go to M, O, and Q, being ftill parallel, and

confequently equally inclined to^he Surface BC ; whencé
it /ollows, that in paíling inro Air, they recede equally
from theif Perpendiculars,- and fo condnue always paral-
lel. But we muft obferve here, that the Rays EF, GH,
TL, which incline towards the Right, when they firfl: en-
ter into the Glafs, are inclined as much towards the Left,
when they come out of it : So that we may faj, 'the Glafs

Chap. 27 - of Natural Philosophy. 217
‘ undoes ihac by the fecond Refractionj which it did by
thefirft. ^

^8. Since Lighc not only íliines, but heats alfcj we 48. Timt au
may here add j that though we cannot pevceive any Ine-
quality in the Añion of lutninous Bodies, but that they prcdmil/"
leein to impelí uni'ormly the fecond Element which fur-
rounds them, towards thofe Bodies which termínate thek
Adion i yet Reafon fhows usj that they a¿t more ftrong^
ly ac fometimes than at others j not only becauíe their
Parts are not all eqiiaU nqr are they always the famc
yíhich aye applied to the fame furroiinding Matter to im-
pelí it ; but alfo becaufe this Adion is at firñ commu-
I nicated to a tranfparent and liquid Médium, the Parts of
i which continually move out of their Places. And this
caufes the fmall Globales of the fecond Element to im-
prefs a kind of Trembling upon the Parts of the Bodies
to which they are impelled by the luminous Bodies ; and
becaufe Heat confifts in fuch a kind of Agitation, it
ftttpws, that all luminous Bodies ought to produce fome
;■ Heat.

i 49. ^owever, it may happen that this Heat may not 45. f'Vhym;
i be at all perceiyable, either becaufe of tire- Weakriefs of
I the luminous Body, or becaufe the Organ upon which
: it ads is'hotter than it. Thus if coming from a Fire we *'«•
expofe our fqlyes in a coid Night to the Rays of the
Moon, we ¡hall find it very coid ,• becaufe in fuch Cir-
i' cumñances, we give more Heat to the Air ‘which furrounds
US, than that that does tq us.

í 40. And as the Sun is very bright, fo it ought to raife Thsf„r-
the moíl: fenfible Heat in us ,• and fo we find by Ex-
[ perience every Day that it does^ nay to that Degree, í*.k.

; that when its Rays are colleded by a concave-Glafs, they
will not only fet combuflible Bodies on which they fal!,
on Fire, but will melt Metals, Stones, and Flints,

2. That «iouble and irregular Re-
frailion of IJland Chryjíal-, where- *

by nac only che oblicjue Rays are
feparaced inco two Parts on the fame
Superficies by a double Refra£lion¿
but alfo thofe that fall perperidiciilar-
ly are_ hálf of them refaíled Jike-
«'ife, is very diíFerent from all thofe
hitherto explamed ; The Explication
of this. you may fee in Nexop. Opr.

P* 33Í*

I. unaoes tbat by the jecona Ke-
fraélioTt) We muft have a Care ot
thinking, that the fecond
Tab, V. Refraítion ib undoes the
Fig. 2. firfta that the Óbjeéf is
feenin its true Place j for
the Ray BQ^ extended backwards
v/ill not coincide with che Ray LI,
buc fall to the righc Hand of it, and
thatfomuch the more, the thicker
the Glafs is. But as to Colours the
fecond Refraífion does indeed undo

fche firíí. íAa A 7 dfi»t nn Jirt.

which

f

218

That the
coloured Body
ís not the im~
mediato Caufe
ef the Senfa-
tion of^Colonr,

5^. That it is
ihe different
Modijicatim
pfthe Rays of
laight that
cattfes the dif-
ferent Senfa-
tion ofColours
in «/•

I

RO H J ULT’s System Partí.

which are very difBcuIt to meit with Fire j as I my felf
have feen.

51. Having fufEciently explained the Nature of Lighr,
and the common Properties of it ,• the firíl Thing that
we obferve concerning Colours, isj that they are not per-
ceived by the itnmcdiate Application of the coloured Oh-
jedt to the Orga?! of Senfation : From whence it follows,
that it does not of it felf excite in us that Senfation of
Colour which we have upon looking on it j for we cet-
tainly know, that one Body cannot a£t upon another with-
out imtnediate Contaél ,• but whatever there may be in
the coloured Object, in which its Colour coníiílsj we
muft thinkj that it aóts thereby upon fome Médium which
it finds, and by that Means a¿is afterwards upon our Or-
ga» of Seníátion.

52. If the coloured Objedh only had been conGdered,
which generally is at reft, when it afifedhs the Senfes, I
doubt the manner of its ading upon the Médium woúld
never have been difcovered, and confequently we lhq|Íd
never have known diftindly what Colour confiíls in. But
if we obferve, that fuch Bodies are not to be jT .-ceived
in the Dark ; and that in order for them to appear co-
loured, it is neceílary for them to have fome Light, the
Nature of which is to be refleded, when it míets with
a Body which it cannot penétrate ^'t is eafy to conclude,
that it is the Light which ads upon our Orgait of Sen-
fation tomake us perceive any Colour, and that the whole
Adion of the coloured Body confiíts in giving it ' fome Mo-
dification which it had not before.

53. This

are the eafie/l of all- -and che moíl
turnad outof a fíraighc Line cowards
Lj by the AiTtion of che refrafling
Superficies; and che reíl of theni)
according as chey exceed each orher
in Bigners, are more dlíficulrl/) and
lefs turned ouc of a right Line, to
che Püints becwixc G and L.

{3.) Thofe Particíes of Light
which are moíl reíraéled, makeu
fmall Ray of a Violec Colour ; that
isj (as is very^ likely) che fmalleíl
Particíes of Light, íeparated from
the reft in this manner, excite the
ftiorceft Víbrations in che Túnica R('
tinas to be propagated from thence
along the íblid Fibres of the opcick
Nerves into che Bra«, there to ex-
cite che Senfation of Violec Colour.
rhe darkeft and the fainteíl of atl
Colours. And thofe Particíes which

I» úovje Modification which it had
poi before ) In order to explain che
Nature of Colours we muft oh*
ferve, ’ '

(i.) Thatic is found by Experi-
ence, that the Rays of Light are
compounded of Particíes diíFerenc
from one another: thar is, •which
are (as is highly probable) íome lar-
ger and fome ImaJler.

(2,) That a Ray, fnch as FE,
falling upon a refraíting Superficies
in a dark Room, is ñor
Tab. IV. refrafled whole to L, but
Fig. y. as it were fplic into a
greac many fmaller Rays,
íí)me of which are refiaíled to L,
ochers of them to fome other Poines
becwixc L and G; That is, (as is
very probable likewife) choíe Par-
tiicles of Light which are fmalieftj

Chap. 27, 0/ Natural Philosoph Y. 218

5:5, This being fuppofed, there cannoc be an eafier rs- "nat the
Way to come ac the certain Knowledge of the Nacure
of Colours. For fince Light is nothing elfe but a partí- ács ofa -Bcdy
calar Motion of the fmall Globules of the fecond Ele-
nient, or at leafb a Difpofition to a particular Sort of Mo-
tion j nothing more is requifite for the underílanding of Light.
Colours, but onl-y to examine the different Modificati-
ons which this Modon is capable of, and to find out whac
there is iii the Bodics which we cali colouredj to caufe
ihefe Modifications. Now the firíl Thing which offers it
felf, and which is the moft íimple Modification, is this,
vi%. That this Motion cannot 'but be weak, íf all the

Rays

are refr.aéted leaílj they make a fmall
jRay of a red Colour; ihac is, the
Djggeít Particles of Light, excite the
longeñ Vibrationsin the Tnnha Re-
tina, in order to raife che Senfation
of Colour, che brighceft of all
Co^si and che other Particles are
alto every one feparaced into fmall
Rays, accflrding to their Bigneís and
RefrangÍD^IIy, in order to excite
intermedíate Vibrations, which raife
the Seníations of intermediare Co-
iours. Much in the íame manner, as
the Vibrations of Air, according to
§ their diffetentBignenesj caufe Seníá-
rions of different Sounds. ^

(4.) The Colours cherefore ot thofe
ím.ill Rays, (ince they are not acci-
denta! Modifications of them, but
cnnnate, origináis and neceíTary Pro-
perúes ot them, confifting fas is
highly probable) in the different
Magnitudes of them, are permaraent
and iinchangeable j fhat is, fuch as
cannoc be altered 'by any future
Refraflions Reflexión, or any other
Modification.

(y.) As the Rays of difFerent Co-
lüiirs begin in chis manner 10 be fe*
parated by che Jtn^lc Refratlion of
ówí Superficies s fo that Separación
is much more compleaied (fo as %^e~
ry cafily to be perceived by our
Senfes) by rha: donble Refrañion
(the Firñ being increafed by che 3 e-
cond) which is made in the cwo
I Síáes of a Triangular-Glafs Prifm,

I (che Phanomena of which are fully
; explaineJ in the Note.': m Art^

; below) . and io che double Refraoli-
i pn made in the Superficies of Glallés
i of other Figures, according as their
Superficies are furcher from being
paraDel to each other, fuch as the
pbjeíl GJaíles cf Telefcopes,

(and chis is the Reafon why they
cannoc be made perfeíf, vix, becaufe
of the Separación df the coloured
Rays. Se¿ the Notes on Chap. xxxiÜ.
.Art, 28.)

(6.) As the Rays of difterenc Co:r
lours are feparaced by che Refradti-
ons of Prifms, and ocher thick Bo-
dies, fo are they hkev/iíe feparaced
in anocher manner, in.very chin
Places of any tranfparenc Maicer,
For ail Places, which are chinner chan
a cercain determínate Thicknefs,
tranfi-nii che Rays of all Colours, and
refleél: nonej buc as cheir Thicknefs
incrcafesin an Anchmecical Progref
fion, chey begin ro refiedt, firíl:,Rays
that are Bine i chen Creen,
Yellow, Red, inorderj and again,
Blue, Green, Yellow, Red , but mora
and more fa'int and mixed i dll ac
lafi, when they come to a cercain
Thicknefs, they refleít the Rays of
all Colours chroughly mixed togecher.
juíl as they fell upon them, and
chefe make Whice. And in chai;
Pare ot the thin Place where íc re-
fledts any Colour, for Inílance,
Biue, ic always tranfmits che con*
crary Colour, viz. Red, or Yellow :
For che truth of all which Pheeno-
mena, lound oiic by numberleís Ex-
periraenes, and for the Calculación
of whac Thicknefs the Place oughc
to be, co refleft particular Colours,
and for che Reafons why Places of
particular Thicknefs refledt parti--
cular Colours in chis Manner: Sea
the eminenc Sir Ifaac Nevoton moíl
clearly difcoiirling in his Opt, Book II.

(7,) All natural Bodies are made
up of very thin tranfparenc fmalí
Piares j which, if they be fo regu-
larly difpoíed, with regard co each
other, Chat there is no Reflexions
pr

2 20

ROH AV LH' s System Part I,

Rays of Light which fall upon an Objed ín a certaiii
Order, and in a cerCain Quantity^ be not refleded bad
in the fame Order, ñor ín the fame Quanticy towards one
decerminate Place of che Médium where che Eye is fixed:
And we are furcj chac chis muft neceílarily happen, if tht
very fmall Parcicles of che illuminaced Body are fo difpo.
fed, as co make a rough and uneven Superficies,- for chen
the Rays which come as it were parallel from che lumi-
nous Body, fall upon fuch a Superficies with a!l forts of
Obliquities, and cherefore are fcaccered and refleded ali
Ways j and chis is che Reafon why che Eye does not re-
ceive the Lighc with its full Forcé ; but only a certaia
Enall Number of Rays are determined by chis Superfi-
cies to come to the Place where the Eye is fixed and
henee we may conclude, Chat chere is fome particular Co-
lour which confitfs only hi the Rougbnefs of the Surface of
the celoured Body, and which gives no other Modification it
the Light, hut only this, that it refleBs it all ways indiffe,
rently in the fame manner as it received it. 0 j,

54- What 54- Now as this is che leaíl Modification of Lighc that
can be ; fo the Body which caufes it ought tqjr.efemble
cmfiftsln. the luminous Body as much as pofllble, that is,'ic ought
to excite in us the Senfation of Whitenefs, which comes
the nearefl; to Light of any Colour. And this is confirm-
ed by Experíence for the whice Colour of Eflamps Sand
is found to confifl: in this, that. ef&ry Grain does thus re-
fled anyRayof Lighc all Ways. For when we look up-
• on any of the Grains with a Microfeope, they have iio
Colour at all, but are tranfparent, like fmall Pieces of
Chryftal of all Shapes, or like lictle Diamonds which af-

or Refraíiions ín their Interdices,
then ihey conditnte a ira»fparent
Body. Bucifthcir Interdices be ío
ídrge, and filled with fuch Matter, or
lo emjjty (proportionabiy to the Den-
llty of the Parts themfelves) chac
there are leveral Kefíexions and Ke-
fraccions made within che Body,
then chac Body is Alto 5

above) Further, chofe opake Bodies
■*.vh¡ch are made up of che chínneft
ímaJl Places qf all, are "Blnck ; and
chpftí chac are madeup of che chick-
ed fmall Plates or of íych as are
or very differenc Thi^cknefi'esj and
are cherefore ficted co rcfíefic all Co-
Jours, fuch as che Froch of Water,
chele are yVhite i and chufe -which

are made up of fmall Places, the
mod of which are of fome incerme-
diace Thíckneís, are rherefore ¿/kí,
Grr-etij Tel¡oiv, of Red» viz. byre*
flefting noc all the Rays of chai
Colour, biic more of chofe chan of any
other Colours ; the greateft Pare oí
which other, they eicher fufíocate,
and by inter^epclng chcm,excinguiíh
chem quite, or elfe they cranímk
chem j whence ic is? chac fome I-i-
quors (for Inflance, an Infufion of
Lignum Nephruicum) appears Red
or Yellow by a reHeñed Lighc j and
Blue by a cranfmicced Light s and
Leaí-Gold appeafs Yellow when
looked upon. buc Oreen or Blue whcü
looked fhrough.

ford

221

Clmp. 27. of Natural Philosophy.

ford fuch a PaíTage to the Light, that they refled it all
Ways in the fame manner as they received it.

We may further conjedure, nay, we may be af-
fiired, that the EJfeuce of Whitenefs mífifls in nothing elfe
hut the Roughnefs of the white Body, if we coníider, that
we cannot make fome Bodies rough, but they will alfo
become vohite at the fame Time, ñor take away their
Roughnefs, but we muñ likewife take away their Whitestefs.
Thus Goldfmiths make Silver by putting it firíl
into the Pire, to take ofF all the Drofs and Dirt which

I foils it; and then dipping it in boyling Water, into which
they cali a certain C^antity of Tartar and common Salt
(which are corrofive Bodies, and proper to make the Su-
perficies of Silver rough and uneven.) And to take off the
I Whitenefs, they do nothing more but rub the Silvei with
! what they cali a Blood-ftone, which is very hard and
L fmooth ; which by preffing upon the Pait it is applied
I tO) muft neceíTarily depreís the Parts which ftick up, and
rai^the Parts which fink in, that is, take ofF the Rough-
nels?

5d. A^we take it for granted, that a white Body does
i not abforb any of the Rays, but that its Superficies re-
® flefts them all Ways indiíFerently, it follows, that we can-
not place the Eye any where, but that it v;ill receive pret-
ty near the fame Number of Rays as if it were placed
any where elíé ; and c^fequently the Body ought to ap-
pear white from what Side íbever it is viewed. But the
Cafe of plain poliílied Bodies, fuch as Looking-Glaíles, is
difFerent; for when they receive the parallel Rays of Lighc
■ from one Side oniy, they can refled them to the other
Side only, where they may dazzle the Eye, but they will
not refled Rays to any otlier Part.

57. As Black is contrary to White-, there is no doubt
but that the EÍIence of Blacknefs confifts in the contrary
to that of Whitenefs. Wherefore, as it is neceílary, ia
order for a Body to look White, that it fhould refled the
Light which falls upon it towards all Parts in the fame
tnanner as it receives it, fo that there can be no Place,
but that a fufficient Quantity of Rays muft aíFed our
Eye : So likewife oughc we to ihink, that in order to per-
ceive Blacknefs, there muft come no Rays at all to the
Eye ; and confequently the Bodies which we cali Black,
and which appear fo to our Senfes, abforb all the Rays in
fuch a manner, that they refled none of them to make
any Impreffion upon the Eye : And becaufe a Body can-
not deftroy the Motion of another Body, but by gaining.

ff. That
Roughnefs is
faficient to
canje White-
nefs»

tVhy a
white Body
looks foi rehén
viewed every
Way.

57 . Of the
Ñatnre of-
Blacknefs»

222

RO H AU LT’s SysteAí

Part

¿teat TTiiiny

Jodies ihat - . .
arenot'Blackt

/o.

• 59 . vny

Wood vohen
ít is biirnt to
a Coal^ tnrns
'Slack.

ic itfelf, it is eaíy td conceive, thac the Parts of Black Bi
dies are very fi?se and hroke», fo as to becafdy Jhaken.
s%.vvhy a 58. And this is confirmed from henee. Firíl^ TJiat
Darknefs, [hat is, thofe Places where Bodies having no
falüng upon them^ can refleít no Rays to tbc
dayeca[fear Eycs^ ' appeats Black. Secondly, Shadows, or thofe Pía-
ces, which, by reafon of the Interpoíition of fome opake
Bodyj do not receive the Rays of Light from the lumi-
nous Body, or receive but a few of them^, appear Black,
Laftly, 'A well-poUfhed Body, which does rqceive a great
many Rays of Lighd but refleds them to the Side op.
pofite to US, appears Black.

59. Thefe Things being allowed, it will notfeem ftrange,
that Fíame which is fo bright, Oiould convert White Wood
into a Black Goal. For it is manifeft, that the W ood has
loft a great many of its Particlesj, which ferved to noii-
riílr the ' Fíame j wherefore the greatefl: Part of the
remaining ones are fo difunited, and ealily íhaken,
that they abforb almoít all the Light that falls An
. them.

60. That all 60. I fay, the greateft Part only are difunite^.írnd ea-

the_Parts of py [Q p,g p^[ Motion, and not all of them , for it imy
happen, that the finefl: Partióles which are on the Out-
fide of the Goal, may be like Down to cover the more
folid Parts, and fuch as are capole of reflecting a fiiffi-
cient Quantiry of Rays of Light: And thas we fee, that
after the Fire has carried ofF ali that it can confume of the
Goal ■, there yet remains a great many Parts which coiii'
pofe the Ginder, which are pretty folid, for they appear oí
a whicilh Golour.

6 t. That,cx- 61. Becaufe the Particles of Black Bodies are more dif-
awlso&í United than thofe of White Bíflies, it follows, that they
otight ta contain lefs of their own proper Macter in the laiT\e Bullí
eigh lefs thefe other. And becaufe the more a Body has of

Beavy Matter, the heavier ought it to weigh, therefore

iban VVhite>

1, Aíppears "Black) This is taken

OUC of Aríjrotle’s JirJi "Book of Colonrs.
Chdp. i, There are Three VPays that
Idlack appegrs to us, Vrbere roe
cannotjee at a!l,it is natiirally "Black.
Or róbele there is no Light brongbt
to onr Ejes, Or rohere the rejieálcd
Ijlght is very yare fmall i and

thrts SbadoTos appear Black,

2, Difunited and eafUy (haken)
And they very eafily and Árongly
inake other Bodies, co which chey

are applied, of a Black Colourj he*
caufe che very fmall Particles of che
Goal, the Number of which is very
great} eafily cover nver che grn0éf
Particles of other Bodies. But this
OpinioH} concerning the Nature 4
Blacknefsi in general is very much
confirmed from henee, vix. ths:
Black Bodies areíboner hcacéd ; and
if wetted, grow fooner dry chiin
Whicej as is confirmed by very cei*
cain Experiments. Sfc Art, 62.

Chap. 27 ‘ o/ N atural Philosophy. 223

we ought to conclude, that cateris parihus of two equal
Bodies, the one Black, and the other White, the latter
ought to weighmore thanthe other-, Wherefore the Wood
ought to weigh more than the Goal j and a piece of Whlte
Marble more than a Piece of Black, of the fame Bigneís. 62. why the

62. Hauing thus explained the Nature of White and Smluiilaed
Black, we Ihall eafily underftand the Reafon why the Rays by a Convex-
of the Sun colleded by a Convex-Glafs, will not hurn at
alh or hurn wiih greater Difficulty White Bodks; but '^'^eJfíerthan^^
eafily kmdle Black Bodies, though they be both combufti- they do
ble. For it is evident, that the White Body which re-
fledls al! the Rays that fall upon it^ is not íliaked by
them, and that the Black Body which abforbs and
choaks all the Rays, therefore abforbs them becaufe
it receivqi all their Motion , by which Means it begins to
grow hot, and at laíl takes Fire.

Í3. Henee we fee the Reafon of a Faót which we 63* Wb
lliould not know but by ’Experience ,■ which is, that
Kfee Bodies weary the Sight, and Black ones refrefh it. s¡ght^ and
Fot wecannot lookupon White, but we muft receive th^ -Black Ona
Impre.<i|j)n of a great Quantity of Rays, which fatigues the
Sight, whereas we fee Black when no Rays come to us,
which refrefhes it.

(14. From all whiqh it follows, that thofe Bodies are
the whitefl which rejfedi all VVays, and with the fame are the rehi-
Forcé, all "the Light mich falls upon them j and on the '^f¡
contrary, that thofe Bodies are the blackefi, which abforb <,//.
the Light the moft that can be. Such we have reafon to
believe black Vehet to be, becaufe the fmall Threads of Silk
of which it is made, are like Briftles, and fo placed as to be
as rough as poffible j wherefore it is the blackefi: Thing
in the World.

óy. As to the Modiñeations of the Rays of Light, 6 ;. of the
which excite in us the Senfation of other Colours j as 'd
Red, Tellow, and Blue, we ought to think that they confift ' ^ °
in this, viz. that the fmall Globules of the fecond ERt
ment, which compofe the Rays that are refleded from
all fuch Bodies, have not fo much Forcé or fo great a Dif-
pofitionto goon in a flreight Line, as the Globules of the
Rays which are refledted from white Bodies, and therefore
inftead thereof, they are fome way turned about their own
Centers^ and fo part of the Forcé which they had be-
fóte to go on in a ftreight Line, is fpent upon this
Motion. Which may be juftified from henee, that we
cannot conceive what other Alteration than this can hap-

pen

224 ROHAULT*s System Partí.

' pen to the Rays of Light-, in paíling thróugh • á trian-
gular Glafs Prijw ,• and yet we Cee, that by going through
this Prif/i!, chey are capabíe of exciting in us theSenfation
of Red, Tellowj and' Blue.

66. But

trhngiiUr Glafs Pfljm) Becaufe
the Experimencs of a triangular
Prlím, are as ic were the Touch-
íione by which every Hypothefis,
and every Theory, concerning the
Nature and Propercies of Colours, is
tü be examined and cried i I íhall
not think it to'o piuch trouble briefly
to enumérate here the principal
Pha:nomena as cbey are explained
by che íamous Sir Ifaac Nexoson all
aíong inhis Opticks. i. Then, che
Rays ot Light cranímicced through
a Priímr paint an Image upon the
oppoíice Wall, diíHnguilhed into va-
ríou.9 Colours, che Chief of which
are, Red, Yellow, Oreen, Blue, and
Violec. 2. This Image is not round,
^ut when che Angle of che Prifm is
abone 60 or 6 s Degrees, five times
as long as ic is broad, 3. Thofe
Rays which make a Yellow Colour,
deviace more from a ílreighc Line,*
tlian chofe which make a Red i and
thoíe which make a green Colour,
deviace more chan chofe that make
a Yellow, and chofe which make
a yiolec Colour deviace moñ of all.
4. If che Priíln, rhrough -which the
Rays are cranímicced, be (b curned
about its Axis, cha: che Red, Yel-
low, Creen, i&c. Raysfall in order
through a fmall Hole upon another
Prifm, about twelve Foot Jiílance,
and be curned another Way j che
Yellow, ó^c Rays, though chey fall
with the lame íncidence upon the
fecond Prifm as che Red do, yet
tney will not be curned upon the
fame Place as the Red, but will be
carried further towards that Pare,
to which the Refradtion is made.
l'urcher, if in che Place of che fe-
cond Prifm chey be received by a
Glafs thac is a lítele gibbous, che
Yelio.v, Green, Rays, every one
in their Order, will meec in a Fo-
cus fooner chan the Red. y. The
Colours of the coloured Rays, well
feparaced, (the manner of doíng
which, you may fee in Nevot. Opt.
p. 5-4, &c») cannoc be deftroyed, ñor
any Way alcered by repeated Re-
fradlions, 6 . The Colours of co-
lüured Rays cannoc be ac all alcered,

by pafling th'rougíi a Pláce that íj
Lighc, ñor by croíTing each ocher;
ñor by the Confines of a Shadowj
ñor by reflefting chem from any
natural Bodies in a Place dark every
where elfe, 7, All the coloured
Rays cogecher, collefted eicher ,by
íeveral Prifms, or byaConvexor
Concave-Glaís, make White, bu*
when feparaced, afeer croffing each
ocher, chey all eschibic their own
Colour. 8. If che Raj» of the Sun,
fall upon che inward Superficies of
the Priím, with the greaceíl Obli*
liquity that any of the Rays can be
tranfmirced ar, thofe thac are refleft*
ed will be Violec, ,and chofe
are tranfmitted will be Red, j^It
chere be two Prifms, che one filled
with a Red Liquor, a^v^the otbet
with a Blue i che cwo rf.lms clap*
ped cogether will be opake, thougli
if they be both filled with a red or
blue Liquor, chey will be tranfpa-
rene when clapped cogecher. 10. All
naturp^'^odies, but efpecially White,
when Gooked ac through a Prifm,
appear co be bordered on one Side
with a red and yellov/ Colour, and
on the ocher Side witb a Vio-
lec and Blue. il. If cwo Prifms be
ib placed, that the Red of the one,
and che Purple of che other, be
mixed on a proper Piece of Paper,
furroiinded wirhDarknefs, chere will
be a palé Image 5 which if ic be
looked upon through a cbird Prifra
ac a due Diñance, v/ill appear don-
ble, Red and purple. 12. So like-
wife, if two Sores of Powder, che
one perfeñly Red, and che other
perfeífly Blue, be mixed cogether,
and any fmáll Body be dawbed
chick with that Mixture, ic will
appear co the Eye through a Prifm,
to have two Images, a red and a
blue One.

Thefe are themoft general Pheeno-
mena of the Prifm j (toreckonup
all the Parciculars which are worch
obferving, would be endleís) from
which ic appears ac firft Sighc, thaC
the Colours cannoc confilí in the
curning round of the Globales oniy,
according tu Cartesy ñor in che Ob*
liquity

Chap. 27. of Natural Philosophv. 225

66. But for the clearer underftanding hereof,- let the, ^6. oftht
Side BC of the Prifm ABC be covered all over with
fome opake Bodyj except the Place DE, tvhere theré Ughtpaf-
is to be a Hole in the opake Body for fome of the
FI, GL, Goming from che Sun FG co pafs through; Tab. y. ‘

whicb, Figij.

riguityof thePulfts of the cbcherial
Matter, as Mr. Hook chought, Mi-
crog, Obfer. 9, ñor in the Light being
tbick and rare or ílower moved í as
the famous ^Barrorv conjcítures?
Ze¿?. la* towards the End. Buc
thefe and all ocher Ph^nomena of
Coloursj are very eafily and clearly
«xplained» by che true Theory of
ihac incomparable Pciíon fo often
ciied.

For Firíl. The Rays of Light
tranfmitted throKgh a Prifm paim an
Imdge t^on the oppojite Wall, dijlin-
gni/hed luto various Coloftrs : Becauíe
thecoloured Rays are íeparaced by
Reirañion, Thus che blue Rays, for
fe Inñance, markedwith

. , *• the prick*dLine,\vhich
Fi^ I . Fig. 2* begin to beífepafaced in
the Sid;^<í ot the Prifm abe (and
alín in We firfi: Superficies of che
Globe of Water abe) from the reft
by the firfi: Refraftion in dd j are fe-
paraced ñill more in be the other
Side of the Prifm (and alio in Co-
rning out of the Globe abrX by a
íecond ReiraíHon 'wwards
Fig. 4. the famc pare in ee : Buc,
Fig. 3. on the contrary, in che
plañe Giaís abef (and al-
fo in the Prifm glo placed in ano-
rher Situadonj the blue Rays» which
begin co be feparated from che reft
in che firfi Superficies in dd, go out
parallel in the other Superficies, che
Kefraítion being made che contrary
Way, Chat is, chey are mixed
again with the Colours of che other
Rays.

Secnndly. This Tmage is not rotmd,
hkt abotttfive times as long as it is
bread \ Becauíé íome Rays are more
refrañed than ochers, and cherefore
they repreíént a greac many Ima-
gesef che Sun like one Imagedrawn
intó a grea: Length,

Thirdiy and Fourchly, Thofe Rays
fohich make a yellow Colonr, deviate
morejrom a Jireight Line, than thofe
which make a Red^ and thofe which
make a green Coloítr^ deviate more
than thgfe that make a Télíoxo, &c.
and thofe which make a violet Co-
deviate moji of all' ^nd{»T-

ther» if tke Prifm through which th$
Rays are tranfmitted, be fo turned
aboutits ^xis) that the Red, Tellow,
Oreen, &c. Rays, falL in order thro*
afmall tiole npon another Trifm
bottt twelve Foot dijfancei and be
turned another Way ; the Tellow, &c.
Rays, though they fall with the jame
Incidente upon the fecond Prijm as
the Red do, yet th^ will not be titrn^
ednpon the famfi- Place as the Red,
but will be carrled further towards
that Part, to which the RefraÜion is,
made, Further, if in the' Place of
the fccond Prifm, they he recehed by
a Glafs that is a little gibbous, the
Tellow, Oreen, &c. Rays, every oné-
in their order, will meet in a Focas
fooner than the l^ed : Becauíe che
Yellow Rays are more refraáted than
che Red, and che Greeii^ than the
Yellow, and che Blueand Violeemofi
of all.

Fifthly and Sixthly. The Colonr
of the colotired Rays well feparated,
cannot be defiroyed, ñor any Way al-
tered, by repeated Réfrafíions, ñor
bypajjing through a light Place, mr
by crojfing each other ^ ñor by the Con-
fines of a Shadow,nor by rejleéling theni
from dny natural Radies in a Place
dark every wherc elfe : Becaufe cheir
Colours are not Modifications arifing
from Refraílion, but immutable
Fropércies belonging to their Na*
ture.

Seventhly. uíll the cbloured Rays
together, collelied either by feveral
Prifms, or by a convex br concave
Glafs, make White‘, but when fepa-
rated rtfter crojfing each other, they
all exhibí t their own Colour : For as
the Ray, before in was divided inca
feveral .Parts by Refradbon, was
Whice i lo by choíe Parts being
mixed togecher again, ic becomes
Wfiíce again j and che coloured
Rays, when chey unice, do noc de-
firoy one another, buc are only mix-
ed cogether. And henee ic is, chac
Red, Yellow, Green, Blue, and Violec
Powders mixed togecher in a cercain
Proporción, are fomewhat Whitifii i
chac is, are of luch a Colour as ari-
f*s troni a Mixture of White and

Q.

RO H jiULT’s SYstEM' Partí.

tvhich, according to what was íaid before, will be refraft-
ed in füch a manner, thac che -Ray FI will tend tíwards
Mj and from thence to N, and GL will go to O, ánd
from thence to- P. Whence ic is to be obfervedj thac
Fl, GL are iherefore turned ouc of the Way in'thig
manner) becaufe che fmall Globales at their entring hito
the Gl'aís , find an calior Pallage chis W ay, thac is towards
the righc Pland, than towards the Lefc- Thus for in-
ílance ; Lee ST V be one of thefe Globales, we niuft

Bliick, and would beentirely White,
if Ibtne of the Rays were noc ab-
forbed : Solikewife if a round piece
of Paper be palnced with ali thofe
Colours diñinct from each other, and
in a cercain Proporciona and chen
tiirned very qiiick round upon ics
Cenrer, thac by che Swiftnefs of the
blotioni all the Species of Colours
may be inixed cogether in che Eye 3
the particular Colours will immedi-
accly vaniili, and che Paper will look
aíl of one Colour, ¿which is a Médi-
um becwixc White and Black.

Eighchly. If ths Rays of the S«w
fall upan ihe inward SHperfcies oj
the Prifth tvith the greatejl Obliqiii-
íy ihat any of the Rays can be tranf-
mitted at> thofe vohich are reflelled roill
be ndet, and thofe which are trdnj-
Tüittedy vpill be Redi Becaufe the
Rnys, being ^olouréd béfore they
were refrañed ac all, and the more
tbey arecapable of being refraéted, che
fooner are chey refleíied alio ; are
íepnraced in this manner.

bürichly, If there- be txoo PrIJms,
the oneft/ed xoith a Red Liqtior^ and
the other toith a TMue^ the twa UnJhiSi
cLi'ppcd to^ether, xoill be opake^ tho*
if they be both fiUed voith a Red or
a 'Bine LiquoY) they roill he tranf-
parent toben clapped together \ Be-
caufe one, of chem tranfmic none
but Red Rays, and the other none
bucBlue, therefore when puc coge*
ther, they can tranfmic none acal!.

Tenthly. ^All natural Bodies, but
efpeiially white oneS} roben looked ft
throiigh a Prifm^ appear to be border-
cd on one Side, roith a Red and Tel~
loro Colorir., and on the other Side roith
a Bine and Violet. Becauíe thofe
Borders are the Extremicies of whole
Imanes, which che Rays of every
Specie?, according as chey are more
Or leís réfráíleda exliibic ac a greacer
or ¡efs diliancefroni the true Place of
^e‘'Óbje<5fí

Eieventhly and 'Twelfthly. If tm
Prifms be fo placed^ tkat the Red oj
the one, and the Purple of the othen
be mixed on a Piece of Paper ftted
for the purpofe and furrormded with
Darhiefs ; there roill be a palé Irna«e,
vohich if i( be looked upon throngh a third
Prlfrn, at a dne dijiance, roill appear
double, Redai.d Purple \ iTo likewiíe,
iftvoo Sorts cf Fonder, the one per-
feélly Red^ and the other perjeéllj
Bine, be mixed together, and auj
fmall 3 ody he áaxohed thick
that Mixture, it roill appear toWhe
Eye, throngh a Prijm^ to have íivo
Images, a Red and a Blue^Sc Be-
cauíe the Red Rays, and Tne Purple
or Blue ones are feparated by an
unequal Refraélion.

Moreover, thirce.enthly. If the Rays
.which are tranfmitted through a gíb-
bous be received upon a Piect

of Paper bej%'e they meet in the Fo-
cus, the Confines of Light and Shadoro
will feem tinged roith a red Co/o«r,
but if beyond the Focus with a Bine:
Becaufe in che former Cafe, the Red
Rays, v/hich are fomewhat leísre-
fra¿ted, are uppermoíl,' but afeer
cfoíling in che Focus, the Blue are
•ío.

Fourceenchly. If the Rays that
go through one half of tíóe Pttpil be
intercepted by ariy opake Body pnt
clofe to the Eye, the Extremities of,
the Objeóls beyond^ will appear ting*
ed roith Colours, as they do through 4
Prifm, but no fo vivid : Becaüíe thd
Rays which are cranfmicted cbrougti
the other pare of the Pupil, are fepa-
raced into Colours by Kcíra6bon)
and will not be diluted by the Mix-
ture of che intercepted Rays, which
would have been refrafled che con-
trary way : And henee ic is, thac a
Body which looked ac through twa
Holesin a Piece of Paper, appears
double, appears tínged with Colours
‘ alió.

thínfc

C/iap. 2^. of Natural PHiLdsópHt.' 227

thinic thac the Superficies AB determines ic to move to-
wards Sj rarher than towards V, and confequently to turn
about its Center in the order of the Letters STVj whieh
it will continué to do the whole Length of the Lina
IM. . And becaufe when it is come to Mj where it un-
dergoes a Refradtion towards the right Hand_; this is a
Reafon why ic íhould be curned about again in like man-
ner j therefore it muft be acknowledgedf that the fmali
Globules which come dut of the Glafs towards Nj are lo
modified, that befides the Difpoíition they have to move
ílreight along, they have a Difpofuion alfo to turií about
their own Centers.

G-¡. What was affirmed of the Globules of the Ray Sj.Thát the
FIMN is to be underftood aJfo of thoíe of the Ray Shadm can-
GLOP and of all the other intermediate Rays. But af-
ter the fecond Refradtionj which is made ac the Surface thefe R^yt.
BC, we find cín the one Hand, that the fmall Globules of
the Ray MN” are turned about in the fame manner as

S ey were at firíf, from a new Caufe ; for the Shadow
1 the Side D flackens che Motion of the Globule M orí '
the (toe Side í and the Rays which are b'etween IMN
and COP being ftronger than the other, prefs upon the
Side Q of the ¿me Globule, and becaufe they move the
farae VVay as ic turns, they quicken its Motion on this
Side; And on che other Hand, we areaíTured, thac the
Globules of the R^ GLOP, have the Rotation which
they had acquired from thefe two Refraétions hindred by
Two Things. Firít, From the Shadow which hinders
them on thát Side on which they were moft flrongly
impelled, and retards their Motion. And, Secorídly, Be-
caufe they are impelled on the other Side, by Rays that
áre ftronger, and which imprefs a Motion upon them,
contrary to thac of their Rotation.

68. Having thus confidered the feveral Alteratíons, and 68. matthe
the Reafons of thofe Alterations which may happen to
the Rays of Light in their Way to che opake Body NP y are vohich
we find, thac the Globules which fall near N are turried
foimdwith agreaterForce, than thatmiith which they are mo-
ved on in a flreight Line ; and on the contrary, that the Tab. V.
Globules of the Rays which fall near P, move on in a
firaight Lihe, veith a greatcrFdrce thanthat with vohích they
turn round their Ceñters. And Laftly, That there inter-
mediate Rays, about X, have pretty near the fame Forcé
io turn round, as to rnove firaight along. , But by Experíence
we find, that we fee Red in N, Blue in P, Yelhw irí X,'

Órange b'etween N and X, and Groen bbtween X and P
2 Whience

.. v

'22?

6<j.Whatths
tolours of co~
/onred ‘BúdicS
(onjíji in.

yo.Thatco-
hnred Bodies
ate in fome
meafttre
fra/ijpareni,

^ X . Tkat the
Sítrfaccs of
colottred Bo-
dies is made
roítgh by co-
louring them.

7i, T^at tbe
ÍParííc/es <?/
bUcif Bodies
are more bro-
ken thiin thofe
of any othet
coloHrcd ‘Sí?-
4¡ss^

ROHAUL^’s System Partí

whence ic appears what the particular Difpofitions of the
Globules which compofe cheRaysof Lighcare^ to excite
in US thefé Senfuions.

óp. Now there are two Things in the Objeds » which
we cali coloüred,- which may caufe the fame Modificati-
ons in the Light, as thofe acquired in paffing thróugh a
Prif/,‘2. Fór, Firft, Their Particles may be fo cranfparent,
that the Rayi of Lighc may penétrate a little VVay into
them> and be refracted^ before they are refledted ; Se-
condly, (and which may produce che fame Effefl, and
be the Caufe of the Colours of different Objedts) Their
Particles may be fo fmall and uneven, that the Globules
of the Rays of Lighc which fall upon them^ may com-
rnunicate fome of their JVIotion cq them^ and by that
Meaos they may be turned round and refledted back, in
the fame manner as a Ball ihrown with great Forcé up-
on the Graís, is ílopped a little by the Spires and turn-
ed round.

JO. Neither can it be doubted, but that fome of tha
Particles of coloured Bodies are really tranfparenL as
be feen by the Help of a Microfcope, in all kinds oCSand,
Flint-ítone, Marbie, Sugatj Silk, Wooh Hair, H^Sjanc
an infinite number of other Bodies.

71. And that the Particles are very fmall and brokcn,
is evident, not only from henee, that coloured- Bodies ap-
pear coloured when viewed all Waf'i but is further con-
firmed from the manner in which Colours are made by the
Djers. For, lince B.aíibWood, Indian-Wood, Indico, Yel-
low-VVeed, will not tindture any Thing with a red,
violet, blue, yellow &c. Colour, unlefs there be lome
Allum mixed with them, we muft conclude, that this pe-
necracing corrolive Body inlinuates itfelf into the Pores
of che Cloth, and dilates them j whereby there is Room
made for the Water to enter tindtured with the feveral
Colours, which íink into the Cloth in fuch a manner, as
to leave fome on che Superficies, which caufes a kind of
Roughnefs, and makes it capable of all the diíFerent Mo-
difications qf Light.

72. Afcer what has been faid concerning Dying, it is
neceíTary to make one particular Obíérvation about Black ;
and that is, that becauíe the Roughnefs, in which this
Colour conlifts, muft be che greateft that can be, to ex-
tinguifh all the Rays j therefore in dying Cloth of a Black
Colbur, Allum and Nut-ga!ls are not fufficient alone j

I» Whith voz cali coloured) See above •✓írf. yz*

but

Chap . 2 ^. of Natural Philosophy, 22^

but there muílbe Vitriol inñeadof Allum:, whichismore
corrofive than Allum j and further:, to make the Vitriol
corrode the more^ they put the Cloths to be died intq,
the Copper, and leave them for fome time in the boiling
Liquor ,• whereas in dying of other Colours, they only
dip the Cloth feveral times into the Liquot;, whichis but
juft warm.

73. Since the Particles of é/ad Bodíeí are the moíl T\.-nn.y
uneven, it is eafy to imagine, that Cloths and other Stufl’s

of this Colour muñ tear and wear fooner than thoíe oí than Z"
any other Colour.

74. Further, if we confider, that the darkeft Colours 74. Why
■require that the Particles Ihould be the fmalleíl: that can

be ; it is evident, that we may eafily make a light Piece dy^^dárter
of Cloth of a dark Colour, becaufe it requires only to Cohms, but
llave its Superficies maderough ; but becaufe it is very
difScult to make it fmooth again,- therefore Cloth of a
chrk Colour, can very hardly be died of a lighter.

^7. Now, when I fpeafc of the Particles of coloured 7 y. it is mt
Bddiew I mean only the very fmalleít of all 3 many Hun- xhif ’sZnfe
dreds ot which may be United together differently, in or- famfc°Ínr^
der to compofe groíler Parts which may be of very dif- AoaW hav^
ferent Figures, in the fame manner as diíferent Buildings xa/^”'‘
raay be formed of Bricks, which are all alike. Thus we
know, that coloured íJfediaMd-wpon the Eyes by their fmal-
kfi Particles, and upon the Toiigue by thofe which are lar-
ger, and compofed of the other ,• whence we conclude,
that Things of the fame Colour have not neceíTarily the
fame Tafie.

. 75. Since there are two Sorts of Particles in the fame 75. Hy eiter.
Body, this fhows us, that if we make any Alteration in
the íimaller Sorr, the Colour muí!; be changed likewife. ofany%cdy
And fo we experience it in Herbs bruifed in a Mortar j and the Cohur u
in Colours which Painters grind upon a Stone, fuch as a/fc.
Vermillion and Orpiment. But if the Body be fuch, that
the fmalleíl: Particles of it cannot be altered, neither can
the Colour be changed ,• as we fee in fome Paints, which
are not fo eafy to be altered as thofe mentioned 3 efpecially
as Herbs, whofe Particles have before a proper Motion
of their own, as being in fome meafure liquid, which
helps tp daíh them againft each other, and to le-
parate them into fmallef Particles than they would o-
therwifebe. .

230 RO H J ULT^s System: PartJ.

77. Hm a yy, From what has been faid concerning coloured Bo.
lo%- particularly concerning a whire Body ,• we may

^sar, rohkh infer, thac if there fall no other Rays of Lighc upbn a
white Body, buc thofe thac are caft upon ic by anothjjt
d:jerif 2°'^/ which has altcady modjfied them, the Rays will
noc be akered a,f all. by the white Body, buc refleél-
ed back to the Eye wich the fame Modificación ; íq
thac che Body. inftead of appearing White, will appear
. of the Colour of thac Body from which ic received the
Rays.

7.8. 4 criri- 78 . We may be convinced of chis by a very curious
eus Experi- Experiment, which ic is noc very difficulc to make. The
’ Way of doing ic is chis. Lee all the Windows of the
Room be Ilute up clofe, excepc a very final! Hole, through
which the Rays reflected from che Qbjeets on tljc ouc-
fide, may enter in ,• then lee the Rays fall upon a whice Cloth,
brany other white Body, and ic is pleafant tofeethe diffe-
rent Colours of the Objeóls which are painted upon ic.
-jQ.whythe 79- Experiment perhaps may raifea Difficulty^ti
'Jamisóf the Minds of fome, who may imagine, that diffefent
aiid difFerently modified, paffing through fame
Í»itnd7hiü‘ Hale, muñ hinder one anocher, and confound their re-
oni w the ípe£tive Actions : But ic will noc be hard to get clear of
í^wrdeprty Hifficulty, if they confider inthe fitfl: Place, the vaíl
Mehethers ' nuinber of Pores rh.ic {here.^s iív,|;he leaít Quantity of
E^eíis.^ ■ Qi- of any other tfanfparent Body, which afFo.’-d a
Paíláge for an infinite number of Rays, if I may fo
^eak, withouc diílurbing one anothsr. But that which
is principaily cp be coníidered, and which takes away
the Difficulcy intirely, is, that the Lighc, or the Co-
lours, do noc confift fo much in aélual Motion, as in
a Tendency to Motion^ or a PrelTure. Now ic is eafy
to apprehend, thac an infinite number of thefe forc of
Actions, diíFerent from each other, may be tranírnitted
through the fame Point wichout confounding one ano-
Tab. V. Hu- inflance, fuppofe a Forcé cqual to a hundre(;l

rigo^o Pound Weighc,' applied to the Point A, of the llfaight
Line AB, pulhing ic towards B, w'ht-re we fuppofe alio,
that there is a Body able to refill: chis Forcé. The Line
AB could noc move at all according to the Direñion of
AB, much lefs can ic bend towards C or D, becau.k
ic is ñraight ; but the lesíl Forcé thac can be, will bend
it towards any Side whatfoever. Thus ifany Forcé in C
pullies ic by E towards D, if ic be buc the Forcé of
pne Pound, it will bend ic towards D : But if we fuppofe
anocher Forcé in D v/hich can reliíl that of a Pound,

tbis

23 í

Chap. 27. (?/ N A T U R A L P H I L o S o P H y,’

this will hitider the Line AB frora bending ; fo that the
Forcé which is at A, íliall tranfmic ics Aótion whole and
entire to B, without being diñurbed by the Forcé which
is at C : And the Forcé which is at C fliall tranfmit its
Aftion to Dj without the leafl: hindring the Continuance
ofthe Aítion along AB. So likewife we tnay imagine
a FoTce at F equal to five Pounds ading upon a Body
at G. 'I'he íáme Point E therefore nray íerve to tranf-
init as many Adions as we wilL v/ithóut at a!i confoqad-
in diera.

80. After what I have already faid ; I have but on,e 80 , That Co-
Thing more to remarle concerning the Diflindion that is
afually made of Colours ,• toz. that fome of them are '

trae or real Colours, fuch as thofe of Tapiñry, and o- and
thers falje or feeming Colours only, fuch as thofe feen
through a Glafs Prifm. But I don’t fee any Foundation for Coimrí
this Diftindion, becaufe the Reality is juft the fame in
~h of them ; For if the Senfation of Colour which we
ewing a Piece of Tapiñry be real í tliat
wñcWwe have in looking through a Priím is as real , ’ for

. TJíni «yfi

the Pfffm is as real a Thing as the Tapiñry. And in-
deed it is the fame Light which caufes us to perceive
the Colours through the Frifm, as caufes us to perceive
the other.

81. If any one, tt^íBpfW^that Diñindion ^

Colours which we have juft now rejeded^-replies ; that

diere is at leaft lome falfe Appearance in looking through aií Caknn.
a Prifm, becaufe we apply the Colours that we fee, to
Objeds where they are not ; To this I anfwer, that the
Fault is not in our Sight, but only in the Judgement
which we make afterwards. And if this were fufficienc
to conclude, that thefe ate falfe Colours j we may
for the fame Reafon fay, that all other Colours are
falfe likewife, becaufe -we equally falfely refer the Sen-^
fations which are caufed in usby them, to the coloured
Objeds.

82. Ñor have they fucceeded any better, who owning

all Colours to be equally real, have yet diñinguiñied thcfn

iato fixed and flying ^ giving the Ñame jixed to thofe /or
which the other called real j and the Ñame to thofe

which the other called ; For if the Eye continué n.e-

ver fo long appiied to the Frifm-, and during that Time

the Light intervene in the fame manner, we ñiall always
fee the fame Colours ■, fo that thefe are no lefs fixed and
d.tir%ble, than thofe of a Piece of Tapiñry.

S^.Tha^
thére is as
little Rtafon,

§’ 3 ’ 41

232 System Part L

^83. r¿aí 8^. All the DifFerence that is to be found in the Ob-
at Senfatíon of Colour, is only thisj

aii kimeev that fome of theni:, fuch as the Prijm, feem to require
*’h "othtr^ íhould be fixed in a certain Place, out of

f- ■ ■ which there is nothing to be feen j whereas ochers, fuch

as Tapiftry appear of the fame Colour, which way fo
ever they are loolsed upon. Howeyer, if we confidei*the
Matter a lictie more clofely, it is certain, that the Priftn,
and the Tapiftry, agree in this : that the fame Parts of the
Tapiftry which refle£t the Light to the Eye when it i$
in ány certain Place, does not refleft the fame to it,
when it is removed ever fo little out- of that Place;
and the only Reafon why we perccive the fame Colour
when we change our Place is, becauíé inftead of thefe
Parts, thole Parts that are next to them, and which are
exadtly líke them, refle¿t the Light in che fame manner.
If therefore che Eye were fixed in one certain Pofition,
from whence it fltould fee fome particular Places of the
Tapiftry of fome particular Colours, and God ího.Sl
andihilate all the other Parts of the Tapiftry, fe, that
they could not at all refledi: any Light in the ’Place
where the Eye is ; it would continué to lee the fame Cor
lours, but if it Ihould change ics Place, fhey would im-
mediately difappear.

84- Oftkí 84. This being T,\:ft-w.tícTft:>ttd,C’'here will be nogreat
Matmenf^ Difficulcy . i>: • explaining thofe Colours which we cali
^Murs}^'’ pha?ígeable, fuch as we obferve in a Duck’s Neck, or in

' ■’ a Pidgeon, or in a Peacock’s Tail : For it is eafie to con-

ceive, that the Parts of thefe Bodies are placed in fuch
order, that thoíe of them which are proper to raodify
the Light afcer one particular manner, are difpofed to re-
fledl it to one certain Place ; and thoíe that modify it

in another manner, refledl it to apocher Place. Thus

if the Eye be in the Place where the Rays come,
which caufe the Senfation of Red in us, then che Ob-
jedf appears Red, and if it be placed where the Rays
which caufe Tellow are reflected, the Obiedl appears
¿"ellow.

85. This is confirmed from henee; thatWprkmenhaye
ITaugeafu found out a Way to make Scuffs of a changeable Colour,
fo/u«r, a¡th by making the Warp of a Light Colour, gnd the Strikivg
^ Colour not quite fo Light : But whac moft refem-
■■ bles the Qbjeóts to which we aferibe thefe changeable
Colours, .are thofe channell’d Tables which reprefent
different Sores of ThipgSj accqrdiog as they are view^d
jfipm different Places : For one of thefe Tables, wheq

233

gé.TScre-
raaining
perties of
Light 4md
ColoKTS can-
njüt be imder-
Jlood» tíil Tse
havecxplain^
tá. the Na-
titrtofViJion^

Chap. 27. of Natural Philosophy,

it islookeddireíftly upon, reprefents a Face ,• when

looked upon on the Right Side, ic repreíencs a Cat, and 011
the Left Side a Skeleton. Thus, as they are diíFerent
Parts of the Table which make thefe diíFerent Reprefen-
tationsj fo likewiíe are they diíFerent Parts of the Pidgeon
which caufe us to fee different Colours.

8í). If afcer what has been faid concerning the Nature
and Properties of Light and Colours, there remains any
further DiíHculty, it will be íblved afterwards, when we
have particularly exatnined the Nature of Viíion: And
this is what I íhall proceed to j which I the more
readily do, becaufe the following Parts of this Treatife
of Natural PhiloFophy, depend in fome meafure, upon
Obfervations made by th,e Help thereof, fo that it is
neceíFary to know all the Circumílances of this Sorc
of Senfation, which is the moft wonderful of anyjihatwe
are poíIéíFed of ; I fhall begin with a Delcription of the
&e, and to avoid Tedioufnefs, I Ihall mention only thofe
l%ngs which belong particularly to this Subjed:,

CHAP. XXVIII.

l^HILST the Eye is inclofed in the Head of any ,,

Animal, the Bones which furround it, hinder us F<¿«río/ífce'
from feeing what Figure it is'ofj but when it is taken y,
out, we find it is round, and fuch as is reprefented in Fig. 'j-
the Figure ABCDEF. FABC is the fore-part of it, or
that which fticks out; CDEF that part which is inclofed
in the hollow Bone of the Head.

2. AB is a tranfparent Part of that particular Coat
the Eye which is called the Túnica Cornea. nea?"^

3- BCDEFA isthereft of this Covering, the Parts of 3. of that

which that are next to A and B, are called the White of
, T-, *0 ítj er the

'theliye. whkeo/íz&d

4. AILB is the Túnica JJvea, in which there is a Hole Eye.

IL which is called the Fupil. nica Uve

5. MN, MN, are certain black Filaments, which are md the i«-

callgd the Ciliary Ligaments there is a certain foft andí'^- ^
tranfparent Body called the' Chrjflalline Humour w'hich is ¡(¿y ¿¡‘j. '

fufpended upon them . ments.

6 . The Space QQQ_¡s filled with a tranfparent Liquor, qúeousHu^’
which is very fluid like Water, and for that Reafon is called mour,

Aqueous Humour. .y.NONP

534

RO H AU LT^s System

í^trt.r.

y, Of ths
Cbcyítalline
Humour*

8. 0/í/íí Vi-
treous Hu-.

0/¿^<?0p-
lick Nerve,
and the Re-r

IQ. Tíjat the
infjde of the
£je is blacka

7. NONP is a tranfpareni: Body of the Figure of 5
Lens, a lítele more convex 011 the Superficies NPN than
on NON, which, becaufe ic is a íictle hard, is called tk
Chryfialline Humour.

8. The reñ of the Cavity of the Eye RRR is filled
wich a üimy Matter, almoftlike the White'of an Egg,
which is more tranlparenc than either the Aqueous or the
Cryílalline Humour, andis of middle Confifteney betwixt
them, (foritcaneafierbecompreíTed than the Chryfialline,
and yet it is not fo fluid as the Aqueous Humourj) and
this is called the Vitreous Humour.

9. DEGH is a Part of the Optkk Nerve, whofe Ca-
pillaments TS, beginning in the Brain, and reaching to the
Eye, form at the Bottom of it a curious Piece of Net-
Work which Phyficians cali the Retma.

10. I purpofely forbear mentioning the Number and
Ñames of the feveral Coats with which the Eye is co
vered, becaufe they are not of any particular Ufe in a
plaining the Nature of Vifion j but I muft not omit|H¡
take notice, thac the Superficies of thefe Coats^’-e all
Black in thofe Places wlaich are over-againfl: che Bouoraof
the Eye.

11. The whóle Body of the Eye is encompaíTed witli
flx Mufcles, four of which are called Right, and the othet
two Oblique. ^^T,"^t-í.f^p,;T»;Eií!’'l'is thought to be the
O.’-iginal of-.'-he leveral Right 3 íufcles, is derived immedi-
ately from the Brain, frora whence it comes along througli
a little Hole in the Bone of the Head, and divides it
felf into thefe Muleles, every one of which is inferted
into fome Part of the Coat of the Eye, fuch as that hete
marked F, in fuch a manner, that of thefe four Muícle',
the Firfl: is above, the Second below, and the other Two
on each Side this Coat. And as the obltque Mufcles bave
their Origin alio in the Brain, their Nerves are bent round)
fo that they feem to come from that Córner of the Eye
which is next the Ear, and one of them fpreads over the
Top, and the other along the Bottom of the Eye, 5 nd fo
crofs the four right Mufcles, and then are inferted into
the Bofie of the Nofe.

la. The Ufe There is no one Mufcle in the whole Body, bui

of the_ jv/fí/- what is fometimes filled with a certain Liquid like very
thin and- finé. Air, which comes to it from the Brain alón»
theNerve whichbelongs toít. This Liquid is what Pliy
ficians cali the Animal Sfirits, which cannot fwell thí
Mufcle v/ithout fliortning it or ieíTening the Length be

•íi, 0/ the
MitfUes of
ski Eje.

cíes of the
Eje,

jChap. 28. e/ N atüral Phil.dsqph^%

twixt the Origin and the Place into which ic is infeited.

Thus whcn the righc Mufcle which is ■ above, is filled
wich Spirits, the Eye nnuft neceílarily be lifted up, and
when the Three other right Mufcles are filled in their
Turns, they ferve either to turn the Eye downwards, or
to the Righc, or to the Lefc Side. But wbat is very
remarkable here, is, chat if theíe four Mufcles be filled
all at the fame, time, they wíH alcer the Figure of the
Eye a little, and malee ic fíatter than it was befóte. Bar
as to che oblique Mufcles, I am not of the fime Opis
nion with thofe Phyficians who fay that they ferve to .
turn the Eye round like a Pulley : I rather think, thac
they are filled both 'together with Spirits, and by thac
Meaos íhortned, and ib they prefs upon the Eye and al-
ter its Figure, in fuch a manner, thac the fore-parc of
ic is made more gibbous, and the hinder-parc funk a iie-^
tle deeper in 5 and this maltes a greatenDiftance between
the Chryílalline Humour and the Retina.

To thefe Alterations of the Eye we may add, that
tl^ Pupil is capable of dilating and concrafting ic felf.

And Üfcus we find, that ic dilates it felf, when we are in
Places where there is buc a little Light, and when we
try to lock at a great Diftance,- and on the. other Hahd,
it contrafls it felf when we are in a very lighc Place, or
look at an Objedt

14. Laftly, we maj^oDm^eTtnaM^th^^o Opich x^.o/as;
Nerves be purfued to che Origin of them, we ¡hall find,
that after they come into the SkuU, they approach nearer
and nearer to each other, tül at lañ their Coats are mixed
together, and they become one and the fame ,• but after-
v/ardí they are feparated again, and then enter into the
very Subñance of che Brain, after which we lee them no
plore. Wherefore tq add any Thing further about this
Macter, would have no Similitude of Truth, unlels ic
were to account for certain Phxnomena which otherw'ife
could not poílibly be explained.

i

CHAP.

23.6 RO H AU s System Partí,'

C H A P. XXIX.

How Vifton is commonly explained,

x.-WhatU ARISTOTLE has íáid nothingin particular as to the
fioTLifláí manner how Vijion is performed ,• for though the
Ariiíotie has Titlc of the Scventh Chapter of his Second Book of the
'Ssaí^v**”^ concenifig Vtjion, feems to promiíe treating of this

Matter fuliy ; yet he fays nothing more of it, but only
this i that the Objed: mull aft upon ‘Cae Médium in order
to have its A£tion tranfmitted to the Organ of Sight.
It is true indeed, that he fays further in the Twelfth
Chapter of the fame Book , that in every Senfation we
receive the Images of the Things, but noc the Mat-
ter; in the fame manner as Wax receives the Impreí^
of the Sea!, without retaining any part of the Seal it Wi
buthere likewife, what he faysis as general and lo»^, as
what he faid in the forecited Place; and the Cpmpari-
Ibn which he malees, does not at all íhow u^ how fo
great a Number of Parts of which the Objedt is’ compo-
fed, can be diftindtly perceived at the lame Time, ñor
howwe ca^iknoY'TÍírSfoSd'rjttí'S'jCi’nce, Bignefs, Figure,
Number, fíítítiqn or Reít of the Objeñs which are in
our View.

í.The opi- 2. The Followers of Ariflotk fawplainly, that he fell
ArTámeliaDs much íliort of teaching what one would wilh to
aUsit vifioD. know upon this Subjeóf ; and this has put them upon
trying to find how his Dodrine was to be underftood.
Thus taking the 'Word Image, which he fpeaks of in the
forecited Place, in the literal Senle ; they affirm, that the
vilible Objed impreífes an Image upon the Air which
furrounds it; thaf this Image impreíles another a little
lefs upon the Air beyond it, and this impreíles a Third,
a little lefs ñill, and fo they go on till there is one im-
preíled on the Chryñalline Humour of the Eye, which
they pretend is the principal Organ of Vifion, or that
Part of the Body which the Soul makes immediate ufe,
of to caule Senfation. Thefe are what they cali inten-
' tional Images or Species ; and in order to explain their Man-
ner of Produdion, they fay, that the Objeds caufe them
in the fame manner, as our own Image is prqduced in ^
Eqoking-Glafs, ■' ’

Chap. 29. 0/ N atural Philosophy, 237

3. Fratn what has beén already faü ic fuSiciently ap- %.That tht
pears, that I agree with Arijiotle himfelf; buc I can by

no Means come in with his Followers in this Thing of expuín thc
<^tKintentional Specks, theNature of whichfeems to me Nauatof _
inconceivablcj and has all along put their Underftandings onaTspcdes'
upon the Rack. And ic is a mere Sophifm to pretend to
eftabliíh their Opinión upon the Inftance of a Looking-
Glafs, becaufe refleded Images are harder to be explain-
ed chan dired.

4. There is no need of mentioning all the Abfurdities 4- The Ah-
coníequent hereupon, in order to ílrow that there is no

fuch Thing as intentknal Specks. Ic íhall fuffice only to ^

obfervej that if They are diminifhed in the manner they

.fay, ic vvill foliow, that when an Objed is feen at ten

Ifords diftanccj the Specks of ic is only as lictle again, as

when it is feen at five Yards diftance 3 that isj an Objed:

of fix Footin Length in the oneCafej will appearof three

Footin Length in the ocher Cafe. Wherefore if the Eye

and the Objed be within five Yards of each other> it •

c&receive but a very fmall Pare of fuch a Specks, and *

conTemently we could fee but a very fmall Part of the

Objec™ but this is contrary to all Experience, for we

can fee fuch an Objed intire at fuch a Diñance, nay, at

a much lefs. If they fay, that thefe Specks diminiíh o-

therwife when the Eye is nearer to them, than vyhen ic

is furcher offq they a Ti^g inani-

mate, and which ads neceflarilyj has ESjjjgpiír Under-

ftanding enough to proporción its Adion lo" as to per-

form the fame Thing at different Diftanqes. Which be-

ing abfurd, it follows, that the Foundation upon which

their Specks is eñabliíhedj is ablurd alfo.

5. Icis notonly without Reafon, but contrary to Rea- j.That p?/,
fon, to affirm, ihat ViJ¡ 07 i is perfeBed in the Chryfialliné
Hmnoiir, and that the Vitreous Humour behind it, is of chryftaiime
the lame Ufe as the Quickfilver "behind a Looking-Glafs, Humour.
w'z. to terminate the Adion of the vifible Objed : For
doubtlefs the Objad oughe to continué its Adion thro’

the Vitreous Humour, which being one of the moíl
tranfparent Things that we know of in the World, can-
not reafonably be compared to Qiiickfilver, which is very
opake. To this we may add, that fince the Chryflalline
Humour is found in both Eyes, and two Species are
formed by ic at the fame Time, if That werc the prin-
cipal Organ of Vifion, it would foliow, that we muft al- /

ways fee the Objed double, when we look upon it with
büth Eyes at once.

6 . This

¿gS RORAÜLt^s Systéiíí Partí,

ítjetflmed Reafoa íhows alfo how falfe the Opinión

*»f/«Reán3. of íomc Phílofophers is, wfao affirra che Retiña to be ths
wmci^dl Organ óf Vijíon.

7. Thaí it is 7. As co che Opinión of thofe who .contend, that chis
h¡ th/°pZ‘cf, Senfttion arifes ffom henee, chat the AStion of the Ohjeñ
toheretheOg- ts cArríed to the Tlace where the O^tick Nervés meet • chis
• conFuted by the EXperierice of Anatomifis-¡ who have

* ‘ found thefé Nerves féparated in the dead Bo’dies of fomc
Men, who, when they were alive, íáw Things in the fame
rnanner as othérs do.

C H A P. XXX.

Of thé Pajfage of the Light througjo the Rumoun
of the Ey¿

í . Sótv the
S7uient Pht^
(ñfophsTs carne
io he rnij^aken
■kpan this Sub-
jeQ of l'ijhn.

3, h if
(kfjicient to
confíder only
Jome few nf
the fe many
Rays xohhh
torne f rom
every Point
of an Obfeéí,
Tab. Vf.

j* Tfjat Jome
of the Rays
¿o to the Hot-
tom of the
Eye vPithoKt
•21 ty KefrafiU
s// ai ail.

T Think that raofb of thofe who háve endeavoTOd
-*■ to explain the Nature of Vifion, have run great
Miílakes, principálly froen henee, beeaufe they atcempt-
ed too many Things at a Time, and did not obferve
ariy Method or O'rder : Their Miñake will be a Help to
US, if, ''eoon obfei;y¿t!,S::,.. .tjját Vjfiou is a Confequence of
the Atítió^ of ,r^'ü'Ojetü"üp5S'’^'‘B5tíi the external and in-
ternal OrgaiS’y we, in the firíl Plaee, inquire, how the
Rajs of Light, ’which are thé- Means hy tuhich a7iy OhjeSs
are feen, are receiied hy the Himours of the Eye,

2 . Let US fuppofe, fot Inftanee, Z to be the Eye, and
ABC the Objeót; there is no Doubt, but that every
Point, that is, every thé ftnallefl: viíible Part of thií
Objed, fends forth RayS all VVays clirough the Air, to
every Place where it can be feen ,• but beeaufe thofé
only which pafs throügh thé -Pupil are of any ufé to
caufe Vifion, we will examine thofe only which fall up-
on that Part of the Túnica Cornea wHich anfwers dired-
ly to the Pupil ; Thus, in order to examine the Adfion
of the Point B, it is fufficient to contider fome few
of the Rays which come froha- this Point, fuch as BD,
BE, BF.

3 . Now beeaufe the Ray BD is' perpendicular fo the.
Superficies EDF, it will not be at all refraíted in paíEng
out of the Air into the aqueous Humour, wherefore it
will continué on in a ftraight Line to H, where falling
again perpendicularly upon the Superficies of the Chryf-

• ' tal-

Chap. o/ N atural PHiLosoPHy.’ 239

talline Humour, i: muft go on ftill diredtly to M; and
here falling again likewife perpendicularly upon tbe Su-«
perficies of the Vitreous Flumour, ic muft go direótly
to the Poinc O in the Bottom of the Eye.

4. But the Ray BE not falling perpendicular upon the 4. o/ffeRj-t
Superficies EDFj, where ic is to pafs ouc of Air into qf
Water, it ought to be refrañed, and to go towards the ihe'^¿ly"aní
Perpendicular EP, and confequently ic will tend tofome hm thty^

Poinc of the Superficies of the Chryñalline Humourjfuppofe

G, which is fotnewhac nearér H, chan it would be wichout „/ <,» ohíea,
fuch Refradion : Again, the Ráy EG likewife, not being agamia
perpendicular to che Superficies GHI, through which jt is Ret”nár
to pafs out of the Agueous Humour into a denfer 'M«-
dimti, ic ought to be refraded again, and go towards the
Perpendicular GR, and confequently to arrive áC fome
Poinc of the Vitreous Humour, as L, which is nearer to M
than if there had been no Second Refradion : Lañly, Be-
caufe the Ray GL is alfo inclined to the Superficies LMN,
through which ic is to país from a denle Médium to one ^
thfeis much rarer, it tnufl: be refradedi and gofrom the
Pe^endicular LT, the Poíition of which is fuch, you '
fee, tíííl: che Ray, by -going from the Perpendicular, ap- ’
proaches towards the Ray BDO j and we may conceiv^/
it refraded in fuch a manner, that it llrall go toy^fie
fame Poinc that the Ray BDO went to, that is^Jo the ■

Point O. So likewif*^iig|gg|iiiliail[|¿e we

(hall find that the Refradións will carf^ii^^m F ta
L and from I to N, aña that at laft ic wilTmeec the o-
ther Two at O. And .lince the Rays which fall becwixt
BE and BF, are not quite ib much refraded as they
themfelves are, it is evident, that they cannot do otherwile
than meec all together in che fame Point O. Thus we
fee, that the Poinc B ads upon the Bottom of the Eye,
in the fame manner, as if the Pupil were of no Breadth,
and as if there were to come but one Ray wich a Forcé
equal to the Forces of áll them that are contained between
BE and BF.

5. Nowif weconfider the, Rays which come from any j. rw ¿s*
other Poinc of the Objed, as from A, we íliall find, that

«11 thofe which enter into the Eye, will be- refraded
in fuch a Manner, as almoft to meet all together in Poínts of tht
the fame Point X. And fo likewife thofe Rays which
Come from any other Poinc between A and B, wiE áfferent e-
íneec very near together in fome Point of the Bottom
[ of the Eye between X and O. So that we may aíifirra
! in general, That every Fohit of the ObjeSi, afis •very near-

6. That tht
Kays tehich
vane from

Ja exañly as
thofe vphich
come from
fome othsr
Soints*.

That ¡f the
£yt cotild no
v>ay be altera
edi the ile-
fraclians

RO H AULT* s System Partí,

ly hut upan one and the [ame Point m the Bottom of tht
^Eye; and on the other Handj That every Point of tht
Bottom of the Eye reieivet very nearly the hnprejjioti of om
Point only of the Objehí.

6. I fay very nearly, not exaflly. For if the Superfi-

^ cíes EDF, GHI, LMN, were of fuch a Curvature, as

Jims Faints, to carry the Rays from one fingle Point, fuch as B, to

anocher fingle Point fuch as O, exaflly^ it would be
imponible for thcm to imite the Rays which come
from any other Point fuch as A, becauíé every o-
ther Point is diíFerently fituated from B wich relpedt to
the Eye.

7 . Now we may obferve, that' if the Obje£t be re-
moved further from the Eye, in fuch a mannet that the
Point B continúes always in the Line BD, and the
Shapeor Difpofition of the Eye be no ways altered; the

anitcvplJ^' which come from the Point B to the Pupil, will not
tóíRecfnaíAe diverge.fo much, or be at quite fo great a Diílance from
Rays ahich each other as they were before- wherefore in entring the
o^7á77taii Superficies EDF, GHI, LMN, they will b(^-
Sartsof D'if. fradted in fuch a manner, as to reunite a little near^to
the Chryftalline Humour than the Point O is. the
■ other Handj if the Objed be removed nearerto the Eye:
beeaufe the Rays which come from the Point B in or-
• der co.país through the Pupil, diverge more than they
did, thPv" RefradI^>rW^.i 7 a.u%j;ljejn to meet beyond the
Point O.yAnd^ne'Übjedf'fiíáy beVo'very near the Eye,
that the Rays 'which come from any one of the Points,
may diverge fo much, as never to unite at all. In all which
Cafes, ’tis plain, there would be no one Point of the Ob-
jed, that would not eíFed roo large a Portion of the Bot-
tom of the Eye and confequently the Adion of each
Point, would be confounded by that of the Point which
is next unto it. ’

g. ofitte M~ 8 . This is what would happetí, if the Figure of the
teratim made Eyc could not be altüred ; but to- remedy all thcfe In-

conveniencies Nature has fo formed the Eye, that it can

become flatter or longer to fuch a Degree, as to adjuft
it felf to the différent Diñances that we would view the
Objed at. Wherefore when we would look upon an
Objed at a greater Diílance than it could be ícen di-
ílindly at whea the Eye is of the ufual Figure, it is
then made fíat by the Help of the four right Muf-
cles, all which ading together, pulí it towards the Bot-
tom of its Ball, and the Retina is by this means near
cnough to the Chryftalline Humour, to be exaólly lo

the

i« the Eyíi in
arder to re-
•unite thcm»

241

CHap, 30 - of Natural Philosophy,

the Place where all the Rays whicb come from any one
Poinc of the diftanc Objeót are reunited. And when we
would look upon an Objeét that is very near, the Eye
is lengthened by the Help of the oblique M úfeles which
encompafs it, and by being fwelledj compréis it j and tben
the Diñance between the Chryftalline Humour and the ,

Retina becomes greater, that the Rays which come from
any fingle Point of the Objed: which is fo neatj may be
reunited in a íingle Point upon the Retina. If, therefore,

■ there remains any Confuíion which Nature has not pro-
vided a Remedy for^ it is only in refpeS: to the Aftion
of thofe Rays which come from an Objedt that is too
near the Eye, at two or three loches diftance, fuppofe ;
but this is needlefs, or at leaft, not neceíláry to be reme-
died ; foras Sight was given us principally to take Notice
of Thingsat a Diñance, and there is very feldom any Oc- '
caíion for feeing Objedts fo very near, Nature has not pro-
videdfor it.

This Approaching and Receding of the Chryñalline g. ■

L%nour with refped to the Bottom of the Eye, is fo ne- E^^ofBirds
ceffa^in order to fee diftindly, that becaufe it cannot
be penbrmed by Mufcles in fome Birds, the Coats oí^nneT.
whofe Eyes are almoft as hard and inflexible as Bon^

Nature has provided another Way. For there ar^pa-
ced in the Eyes of fuch Birds certain black FiLiffients,
that are not in the Ammals, by

which the Chryñalline Humour is connec^Siíffla the Bot-
tom of the Eye, and by which it can be made to draw
nearer to, or remove further from the Retina.

• 10. It is obfervable, that the firñ of the three Refra- lo, A ff"

ñions which the Rays of Light undergo, in paíTiug thro’
the Humours of the Eye, is not to be found in Fifhes
who live in the Water, becaufe the Rays are already in Filha.
an aqueous Mediuin, when they begin to enter into the
Eyes of Fiihes. And this feems to be a Reafon why the
Want .of thb Refraftion íliould be corapenfated fome
ochar Way. And fo we find it is ; for Nature has made
the Chryñalline Humour of Fiflres Eyes more convex,
infomuch, that it is almoft as round as a Globe, and v

not of the Figure of a Lens, asitisin other Animáis. »

II. As moft antienc Perfons grow lean and thin by n. That the
Age, fo their Eyes grow flat and more funk chao when “/
they wereyounger. Now in this Figure of the Eyes, the
Rays which come from an Objed very near, come to y'tr cmftfci
the Retina befóte they are reunited j wherefore they im-
prefs but a confufed Image upon it j fo that it is inápefli-

R ble

242

ROHJULT^sSv&^rnu ‘ Partí.

ble for fuch Sort of Eyes to receive any diftinfl; Image,
excepc wheíi the Objeft is at a fufEcient Diftance.
lí.Thatthofe 12- On thc othcr Hand, fome Perfons haveby Nature
EyeswhUh Eyes thac are longer and more gibbbus than thofe of
‘andBkk‘'oÍt 3 which the Diftance betvvixt the Chryf-

rece!v‘im°y‘ ’ talline Humoun and the Bottom of the Eye, is likewi'fe
a cmfufed greatcr than ufual : In thefe, the Rays which come from
oíjc¿h°that Point of an Objcdt further oflF than ordinary^ are reuni-
are at a Di- ted alfo, bcfore they come at the 'Retina-, and then are fepa-
jianit. again, fo that they fpread themfelves 3 little upon the

Bottom of the Eye. Whence it comes to país, fliat
thefe Sort of Eyes can receive oniy a confufed Image
of Objeñs that are at a Diftance; and have a diftind
Image of thoíé only that are near.

(■

C H A P. XXXI. ^

0 )^

hxljat we mean, when we faj, that the Images of
tA OhjeSls are imprejfed upon the Organs of

I Ttoiír- clearly underftand, that every
Jeñima/et.of fingle Poíot of the Objed ads upon one finglc
viable dije¿?.f Poitit only of the Bottom of the Eye which anfwers
‘tMbrReúm. '^íredly to it ; and on the other hand, that every Point
of the Bottem of the Eye receives the Imprcííion bf
but one I^oint only of the Objed ; it is not difficult to
conceive that the whole Objeít ads upon a certain Part
of the Retina, which is as exacUy of the fame Shape
with ir, as could be drawn upon a Cloth by the moft
skilful Painter. We can yet further conceive, thac this
Part of the Retina does ftill more perfedly refemble the
Objed, becaufe it receives as many different PreíTures
/ in all its feveral Parts as there are diíFerent Colours, or dif-
ferent Degrees of Light in the feveral Parts of the Ob-
jed, And becaufe we cali that an Image, or a Species,
which has any Refemblance to the Thing w'hich' it re-
prefents, we cali that Part of the Retina upon which
al! the Rays of the Objed fall by that Ñame, and
láy, That it i??/prejfes its Image on the Bottom of the Eye-

3. There

Chap, 3^.' ^ N ATURAL Philosophy, 245

2. There is no need of íéarching after any other Re-
femblance in chisimage, than what has been mentioned.¿^j„“‘'|-'j'„
For if we would make any further Comparifon betwixt the objaJ,
it and the Objed, we fhall find chem very difFerent. And

firft, herein they difFer, that a Body is always repre-
fented by a Superficiesj and Ibmetimes a Superficies by a
Line, and fometimes a Line by" a Point : Secondly, The
Situation is difFerent, for the upper Part of the Objed.
is painted upon the lower Part of the Eye, and the righc
Side of the Objed upon the left Side of the Eye, &c.

Laílly, They difFer in Magnitude, for a very large Objed
is repr'efented upon a very fmall Part of the Eye.

3. And the further diftant the Objed is, fo much the S'
lefs is this Part of the Bottom of the Eye j as is evident

in the Figure of the Eye C:, where the Space HL which fnm the Eye,
receives the Image of the Objed FG:, is iefs than -the ‘heiejferiíi!*
Space DE on which the Objed AB^ which I fuppofe T^ viuf
equal to FGj is imprefFed,- and this very nearly in the Fig. i.
fame Proportion, as the Diñance of FG from the Eye is
reater than the diñance of AB.

Whoever confiders ever fo little of what we haye
berore laid down, concerning the Nature of Light anj bythefe im»-
Colours, cannot but be of our Opinión, That the Lda- ie
ges of ObjeSs are in this manner imprejfed on the B í^m ofb"”'
the Eye : But he may be further convinced of it^íoai Ex-
perience ^ forif, r^|MÍ|||gi(pi||MlB|||all the/j/Vindows of
a Room, over-againft which are f^s^^jínght Objeds,
we make a Hole in the Window-Shut, and place in it tha
Eyeof an Anima!, freflikilled, firíltakingoff neatlyallthe
Membranes which the Bottom of the vkreous Humour
is covered with, and puc an Egg-Shell in their ftead to
hold' this Humour in, we fhall fee upon the Egg-
Shell a diílind Pidure of all the Objeds that are with-
out.

5. But becaufé there are fome Diflficulties to make this ‘"'"F-
Experimenc fucceed well,- I have thought that the íame ‘tZfame^plr-
Thing might be done, by making a large artificial Eye,
which I accordingly tryed ; The opake Coats, or Tunicks,^
were all made of thick Paper, except the Retina, which
was made of a very white thin Piece of Vellum ; in the
Room of the Túnica Cornea, I put a tranfparent Glafs, and
infteadof the Chryftalline Humour, was a Piece of Chryl-
tal'of the Figure of a Lens, but more fíat than this
Humour ; for lince 'there was nothing in this Machine
but Air, in the Places of the aqueous and vkreous Hu-
mours, a little lefs Convexity was fufficient to produce
R 2 the.

•244

ROHAUL'T’s System Partí.

the Refradtions required ; And becaufe it was very diffi-
cuk to flacten or lengthen this artificial Eye. in the man-
ner the natural Eye is done by the Mufcíes. I placed the
•Véllum in llich a manner, that it could be moved back-
ward or forward, at Pleafure.

6.JT0W t(¡ fte 6. This artificial Eye being Ib placed in the Window
oí’ifí ia ® Room, that the Glafs which reprefents the Tu?iica
Ihii artificial Comea, may be direftly againft fome Ohjeds that are ve-
•Ejí' ry much illuminated ; we Ihall not only fee the the Images

of them impreíTed upon the Vellum, but we rnay alfo
obíérve all the moft minute Pardcularities, which we before
colledted from Reafon. Thus we may obferve,

y. The firft y Xhat it is at onc particular Diftance only of

o fervamn. Vellum from the Chryftal Lens. that the Image will
appear the moft diftindt that is poífible.

8. Thefecotid g. Secondly, That this Image is not fo diñindt in the es-
Ohfiervation. Parts, as in the Middle.

‘ji. 'rhsduyyd p. Thtrdly, That if the Vellum be totí near theLens;^
Obfervation. Image will bc lefs, and very much confufed.
lo.Thefonrth ¡ :[o. FouTthly, That if it be too far, tho Image will?>be
obfervaitm. confufcd likewiíé.

vf- Fifi bly, That the diftind Image of any Objed, is
fo iriuííh the lefs, as the Objedt is more remóte.

12. ha^ly, If a certaitrDiftance berween the Lens and
the VellumV be rgí^Smx^'^iíai^'í^'icina Image of an
Objed at a mSííate Diftance^ the Vellum muft be mo-
ved alittle nearer, fo that the Diftance of the Lens from
it may be lefs, if we would have a diftind Image of a-
another Objed, which is at a confiderably furcher Di-
ftance.

13. Seventhly, When the Vellum is ata proper Di-
Mííon. ftatice, to reprefent diftindly an Objed which is at a

great Diftance, fuppofe an Hundred, or Two hundred
Yardsj therc is no need of altering it, inorder to repre-
fent, as diftindl as is poífible, any Objeds that are at a
ftill greater Diftance.

1 4. Eightbl)’, The nearer the Objed is to this artificial
Eye, the further muft the Vellum be removed from the
Lens.

r^. Ninthly, When the Objed is too near this arti-
ficial Eye, it is impoffible to get any diftind Image,
let the Vellum be removed to what Diftance ' we
will.

11. Thejifth
Obfervation.

12 . The (ixth
Objervation»

1 5 , The fe-

t^»The eigkth*^
Obfervation,

1 5’. Theninth
Obfervation,

16 ■ It

Chap. 31* N ATURAL Philosophy.’ 245

1(5. Ic is to be obferved, that in thofe Cafes where any Thr. d!f-
Alteration muíl be made in the Eycj in order for the h {ZZl^Usar.
mage to become diñincS, this Alceration is much lefs in tijicUi Eya,
the Eyes of Animáis, the Coats of which are flexible,
than in this artificial Eye. For in Animáis, the leñgch-
ning or íhortning the Eye being always attended wich a
greater or lefs Convexity of the Cornea, the Figure of
this Coat contributes its Part in producing that EfFed
which in the artificial Eye wholly depends upon the Length
or Shortnefs of it. Thus, if when the artificial Eye has re-
ceived a diftinót Image of a diftant Objeóf, another Obje£b
be placed befóte it at fuch a nearer Diílance, that íh order
to have the Rays which come from every Point of it rc-
united, the Eye ought to be made One hundredht Part
longer than it is j the Vellum muíl be removed juíl lo
much further from the Lens : But in a parallel Cafe of
the natural Eye, it is not requilite that That fhould be

lengthned a hundredth Part of the Whole, becaufe the

^múca Cornea being more gibbous than it was before,
greater Rejrañions, and fo maltes the Rays reun¡te'(
fooiSr than they would otherwife do.

17. The Image of an Objed l im prefled on the EYe;^- 17. rw tU
an Anima!, being received in a Place where the Cf^la- Capiuments
ments, of which every Optick Nerve is compofeSir meet

each ocher ; it is ye^iiiiif^^iyilmglllIglFis Im''!^is ib im- trmfmíc the
preffed, that the Raysdonotmove^l^íJCapillaments oy'e^íftll*
lide-ways, but always fall diredly upon the Extremicies of ‘ '
them. To which, if weaddj That the ImpreíEon which
is made upon the Extremity of every one of thefe Capil-
laments, is communicated from one End to the other,
we may conclude, that the Image of the Objed; is tranf-
mitted intire to that Place where thefe Capillaments end
in the Brain.

18. And becaufe we have no Senfation, when thoíé iS, That the
Parts of the Body are any way affeded, in W’hich there

are no Ncrves,- it is very probable, that the Nerves are the Sattiper-
neceíTary to Senfation. And becaufe we have no Senía-
tion likev/ife, when any Objed makes an ImpreíEon up-
on a Nerve, if its Communication with the Brain be hin-
dred, or if the Braiii it felf be afíéded with any particu-
lar Diflemper ,• therefore it is reafonable to think, that the
Nerves are not the immediate Orgatis of the Soul, but
that they are fo formed by Natute, as tO- cranfmit the
ímpreffion which they receive, to that Plací \ in the Brain
where the Origin of them is, and where pró ilably the im-
mediate Organ of the Soul’s Senfation is.

R 3 J9. How-

\

24 ^

t^.That
thert is a
Paré of the
Urain xohich
is the prindr
pal Orgart of
the SokI,

as. ^ Conje-
élsíie about
.the Continua-

Capi/Umenx^.
pf the íwo ?
Optick (
Nerves,

Tab. Vil.
Fig, z.

/

RO H jí ULT’s System Partí,

19. However. we may further obferve. that there be-
ing Two of a Sort of almoft all the Parts of the Brain,
they feannot all of them indiíFerently be thought the im-
mediate Orgati of the Soul. On the contrary. ic is high-
ly probable^ that fince we have but one Seníátion only,
though two Impreffions are made by the Objed: upon
the external Orga 7 is of the Senfes which are aíFeíted,
that there^s likewife one particular Place in the
Brain' where thefe two Impreffions meet. Which that
Place is, may be very difficulc to determine j but whe-
ther it be that fmall Gland which Phyficians cali the
Conarium,^ox whecherit be any other Part of the Brain, it
is hardly to be conceived how they can thus unite, with-
out fuppofing fomething equivalent to whac is nov?
faid.

20. Belides the manifeft Refemblance which there is
betwixt the two Eyes j I imagine there is another yet,
which cannot be difcerned by the Senfes, which coníifls
in this, that the Numbcr of Capillaments in one Opjjjll
Nerve, is.equal to the Number of Capillaments M-.me
"ther Optick Nerve. Thus (to make the Thing eaíier)

fuppofe the Optick Nerve of the Eye A to ccíntain
pillaments, the Extremities of which are CDEFG j
it IS rMí^rable to think, ^ha t there is the fame Number
in the Nen^of jpM|iNlMiMMM».mities of which are
HIKLM. ^ü^gine alfo, that the Extremities E and K,
which are in the Middle of the Reít, are exadlly at the
End of the Optkk Axes, that is, at the Ends of the Lines
TE, VK, which país through the Ceiiters of the Pupil,
the Chryftalline Humour, and the Body of the Eye ; and
that the reft are placed ib regularly about thefe, that we
may take íéparately all the Capillaments of one Eye ii)
order, and afíbciate them with thofe in the other Eye ta-
jken in the fame Order, fo as to make up a great Num-
ber of Rairs, which may be called Sympathetkk ; Thus
beginning with the Capillaments C and H, which are
mofton the LeftHand, I make them the firíl Pair^ the
other Pairs are DI, EK, EL, GM. I am alfo of Opinión,
that each Pair of Sympathetkk Capillaments end in * the

Nerves meec in che Brain or no, ít
isevidenc, chac two Images of every
O^jeci: impreíTed upon chofe Capil-«
lamencs muft be feen in the very
fame Place (becaufe che Optical Axes
m^eec each ocher) chac is, muft be-
come one; and chcrefo.re che Objeá;
apjiears fingle»

fafn®

I. /n the fame Pahit of that Part
pf the ‘Brain) This Conjeílure is noc
yet conhrmed, by cuccing open the
Brain. Buc be cloacas ic>viil; cheCa-
pillapiencsCH, plí ¿K,
Tab. VII, «^c. 'njay veryproperjy
Tig, 2. "he calJed Sympathctick,
Forwheiher'che Pairs of

Fig. 1.

Chip. 31- e/ N A TUR AL Philosophy.' 247

fame Point of thac Pare of che Brain which raifes a Sen-
’fation in che Soul ; as you fee in che Figure, where che
Pair CH meec in che Point O of che principal Orgafi
X, the Pair DI in che Poipc P, che Pair EK in che Point
Q, the Pair FL in che Point R, and the Pair GM in
the Point S.

21. This being fuppofed, I conceive that when we 21. Ho» (fe
would look upon an Objeft, we turn our Eyes to ic
in fuch a manner, that the two Optkks Axes meet at mediate Or-
the Point which we fix our Actention principally up-á'”
on. Thus the Rays TE, VK, coming frora that Point, ^ab. vn.’
and falling upon the Sjmp.athetiek Capillaments E and
K, the two Itnpreffions which chey make there, are re-
united in one Point only, viz. in the Point So
likewife the Pare of the Objed which is on the right
•Hand, fhakes the Sympathetkk Capillaments D and I,
the Impreffions of which are carried to P. And again,
the Part of the Objed which is on the left Hand ads
raon che Sympathetkk Capillaments F and L, and their
IS^ffions unite in the Point R, and fo of che reft.

So cSiC though there be two Images impreíTed upon the
Eyes, yec there is but one imprefled upon that Part
the Brain X which we here fuppofe to be the impa?^
ate Organ of Viíion.

which Zí.yfnevi.

well un-

buc be a írill giHüer Surprile^ ChryfiaUme
and almoft all Phyficians Ihould

22. Whac has bee
vifible Objeds impfi
derftood j it cannot
that the Arifloteliam

be fo miftaken, as to affirm, that thefe Images are im-
preffed upon the Chryftalline Humour, and go no fur- 0/ vípm.
ther i for ic will eyidently appear, thac the different Iro-
preíTtons of the diverfe Points of the fame Objed, are all
confufed there.

\

R 4

C H A P. ■

24 ^

I» Jf/iatis
meang by Vi^
Jion,

i. fíbw the
immaíjfiíil /-
jna^e of the
• i's

jorñ7eífíS':^e
ÁoftU

g. Whenoe it
fs that l^is J~
inane is jv
cUar,

4^. That the
hná € of a
y ag a
^¡{iance
bif”ht to he (ti
clear as that
vf a Body
Sfhhh is near^

ROH AUL T^s System Part I.

CHAP, XXXII.

B.OW Vifton is prformed^

A FTER having traced the w¿z/í«¿?/Image of the Ob-
ject, or the ImpreJJion which it makes upon the ex-
ternal Organs, to the Brain, I come next to explain how
this raifes in us an immaierial Image^ or that Senfation in
which Sight properly confifts, and to Ihow the Reafons
why it Í3 clear and difiinói ; and alfo how we perceive the
Tlace, Situation, Dijlame, Magnitudes Figure, Ntmber, and
the Motion or Refi of fuch Objedts.

2. In order to underñand how this mr}^aterial Image ■
is formed in us, I muíl remind you of a certain Truth
which has been fufKciently demonílrated befóte ,• and that
is, That fuch is the Nature of our Soul, that partici^
Motions of the Body to which it is United, are the_(^-
cafionsof particular Perceptionsin it : NowdiíFerenúlü’arts
Objedt, ad diftindtly upon different Parts of the
•|n of the Eye, and their Impreflions being tranf-
mitte^to that Place of the Brain which is the principal

h epd, that the Soul
muíl haveVyiw/^üíra^^WTSñBnS raifed in it, at the
fame Time^üS without any Confufion, as every oneof
’them excites different Motions.

3- It is manifeft alfoj that this immaterial Image, ought
to be ib much the more vivid or e/ear, as the Objedl
lénds forth more Rays of Lighc which are received by
íhe Eye 3 for by this means the Impreííion made upon
the Qrgan will be fo much the ílronger. And the Large-
nefsof thePupil contributes likewife to this Clearnefs, bc-
caufe it aíibrds Room for more Rays that come from the
fame Point of an Qbjed to imprefs the Inage on the
Bottom of the Eye.

4. It is true, that if we confider the Aftion of one
Point of the Objedl only, we muíl iay, that the Senfa-
tion ought to be weaker or more obfeure in proportion to
thegreaterDiftanceof theBody, becaufe the Rays of Lighc
•which come from one Point of it diverge, and therefore
fewer of them enter into the Pupil when the Eye is far ofF,
than wheh it is néar. But we ¿now that one Point of
the Object does not a¿t alone, but always adts in com-
pany with a great many others, and the whole Image of

the

249

le

;hap. 32. of Natural Philosophy.

Objeít is impreffed upon fo much a leís Space oií
le Retina, as che Diftance of che Object from che Eye
greater. Thus if one vifible Pointj ac che Diftance of
m Miles» fend to che Pupil but half che Rays thac it
vould do if ic were but at a Mile diftance only» chis is
nade good by fome other vifible Points that are near itj
vhich fend their Rays upon the fame Capillament of the
d^tick Nerve, where one fingle Poinr of a nearer Objedfc
ffould fend its Rays j wherefore the Vifion ought to be
is jirong and vivid.

To chis we may add, that becaufe we open the ¡.wiydt-
Pupil of the Eye a litde more when we look upon Ob- objeSt
¡eás that are at fome diftance» than when we look at
thofe which are near,- therefore we take in more Rays
from any Point than we do when the Pupil is noc fo
wide» and thismakesche Senfation more clear. And thus. -
we lind» rhac a Mountain looked upon at fome diftance ''
does not appear of fo dark a Colour as when we are
nesterit.

As to che Dijli?í&7tefs of Vifon, that evidently de- 6.;
pend^pon the Refraótion of the Rays ^ and ic is then
as diftindl: as poffible» when the Refraóf ion is ib made, as^
that all the Rays which come from one and the
Point of the Objecft» meec together exactiy in on^nd
the fame Point of d^^Romn^^^^he E'yn But
ibis never is precifely lO^aiHtnnoi^^ lKs wc/ ich come
from that Point of the Objedh which is a?!!ne Extremi-
ty of the Optical Axis ; for it is evidenc» that thofe
Rays which come from the other Points» are reunited
fo much the leís exadliy one than another» as they are
more diftant from this Axis ,• wherefore we cannotatthe
fame time have the moft diftinót Senfation but in this
Place alone» and che reft will be more confufed.

7. This being fo, ic followS from whac was before de- ^
monñrated concerning the confufed ímpreflion of an MenfeeoL

Objed: that is near, on the Eye of an Old-Man •, that
herauft fee fuch a near Objedrvery confufedly ; and thus
we ílrall efcape the Error of thofe» who are of Opinión»
that the Confufedneís in the Sighc of Old-Men» arifes
from henee j thac the Faculty of Seeing, or the Senfe of
Seeing is weaker in them than in ochers. And indeed ic
is very furprizing, and very lucky, thac at a time when
the Doíftrine of Refradlions was noc at all known» A-
rfotle Ihould hit upon faying» thac if an oíd Man had
the Eye of a young Man» he would fee as the young
Man dpes j which is the farne Thing as to fay , thac the

' • ' ■ ' Fault

250

ROH A UL 'T's System

Partí

í, /orne
Perjons Jee
€hje¿is that
are at a Pif-
tance .cmfu-
fidly.

O. Z4notheY
iatife of the
Pifiinéinejs
»/ Víjíon,

S

10 . Wf^y 05 -

vohofe

Parts are of
dífferent Co-
iourst appear
at íi Vifance
of the fame '
ColoHT,

1 1 . fíow roe
refer ouf Sen-
fation tfí ex-
ternalTkings,

Fault in the Sight of an oíd Man, does fiot arife from
Defed in the Faculty of Seemg, but only from fome De,
fe£t in the Organs.

8. On the other Hand we are aíTured, that thofe Per-
fons, wholé Eyes are longer and more gibbous than ordi
nary, receive a diílin£t Impreffion only 'of thofe Objefli
which are near ,• and a confufed ImpreíEon of thofe thai
arediílant: Whence ic is eaíy to condude, that fuch Per
fons muft fee Objeds that are near them diftinótly, anr
thofe that are at a Diñance confufedly.

9 - The Dijlméíftefs of Vilion depends alfo upon tk
Largenefs of the Space which the Impreffion of the Ot
jed takes up in the Bottom of the Eye, • where there ouglii
to be at leaft as many Extremities of the Capillaments oi
the Optkk Nerve, as there are different fenfible Parir
in the Objed which fends forth the Rays, in order foi
every one of them to make a diílind Impreffion. Foi
if the Rays which come from two different Parts of tk
Objed, meet together in two different Points of the *
Capillament, it is the fame Thing, as if tháy met
Point, becaufe they cannot communicate two d^-iterem
Motions to this Capillament at the fame time And
‘ e Reafon why Objeds, that are at a very great Di
ftaíii^, becaufe their Images are irapreíTed on a lefs Space,
are feeii^but

>jed be compofed of

great manjTáífferent Parts which are of different Colours,
it is evident, that if feveral of thefe Parts ad together
upon the fame Capillament, that which is of the brightefi
Colour is the only one that will be feen, becaufe the
Capillament will receive the Impreffion only of this Part.
And thus we fee that in a Meadow where there are a great
many white Flowers mixed with a vaft Number of green
Spires of Grals, at a Diftánce ic looks all White.

II. If ic had never been obferved, that we fometimes
have no Percepción, when we would have fome, and at
other Times have a Percepción, when we would not, wc
ffiould not have been fo ready to have conneded our
Judgement with our Seníation, and Senfation would onl
have been fimple Perception : But when we had once
made this Reflexión, our Senfation muft neceffarily be a
compound Perception ; And if we had been more wary
in our Judgement at firft, lo as not to have aílented to
any Thing of which we had not a clear Perception, all
that we could plainly have inferred, is, that fomething
cpncurrred v/ith us to caufe Senfatioq. But haying been

di&

hap. 32. o/NaturalPhilosophy, z^r

:ently accuftomed from the Beginning, and over-hañy
our Judgement, we have drawn a diíFerent Conle-
lence j and looIc upon che Senfatiorij which now iipon
ore macure Deliberación, we acknowledge only as ati
cidencal Mode of exifting, co be ‘without us> and chere-
re we refer ic co external ObjeSls j and we have fo of-
nmade chis JudgemenC, thac we are accuftomed co do
withouc any Difficulcy, and withouc che ieaft Sulpicion
f its noc being conformable co Truch.

12. We have been confirmed in chis Errour abouc Vi- ü- -Amthe^

» by anocher Miftake. We obferve, thac when an

lake Body is puc between che Objeíl and our Eye, we
len ceafe co fee ic : From whence we bughc co con-
ude, chaC che Thing which concurs wich us co excite
¡enfation, is beyond the opake Body, and being no longer
ble to adir upon our Organs, we ceafe co have the Sen-<
ition we had before. But inftead of reaíbning in chis
nanner, we imagine, thaC the Senfation which we have
)f fehr or Colour, thac is, the Light or Colour which
re pteeive, is beyond Chat Body, and fo carrying our
magi^ion as far as che Objedl itfelf, we go as it
ferc out of our felves, along the Line in which we^
eceive the Impreflion of the Objedt, and afcribe
nvn Senfation co ic, thac is, the Colour which w^er-
:e¡ve.

13. The fame Thing thac leads us to whole

Senfation whida we have of an Objeót co fomeTTnng without itaation of
US, leads us alfo to refer all the particular Senfacions of an oíjeí!^
which it is compofed, in the fame manner, in ftraight

Lines, according to tl?e Diredion in which we receive
the Impreffions from diffefent Parts of the Objed : Thus
the Impreflion which is made in the lower Pare of the
lottom of the Eye, coming to us in the higheft- of ali
the Lines by which the Objedraifes any Senfationdn us,'
is along chis Line thac we refer the particular Senfati-
on which arifes from it. So likewife we refer to the
oweft Part of the Objed, that Senfation which arifes Irorri
the Impreflion made by it, on the higheft Pare of the
Bottom of the Eye. And henee ic is, that though the
whole Image which the Qbjed impreífes on the Bottom
of che Eye be inverced, yet wlren we look upon the Ob-
jeft through a íjmple uniform Médium:, chis hinders not
but that ic appears in its true Situación, thac is, the
immaterial Image makes" the Objed appear to us as
je is, ■■ ■ ■

>

J4. The

252

ROHAULT>s Syste

M

Partí

14. Hom «Oí The Knowledge of the Dijlance of an Objeft

ftrcetve its
Vijiance.

as well as that of the Situation of itj depends upon 0111
referring our Senfation to fomething without us. Foi
our regard being chiefly upon the Pofition of the two
optkal Axesj and the Motion of the righc M úfeles ol
our Eyes by a natural Way of Reafoning, 'Ihowing u
very nearly, the Relation or Inclination which thefe m
Axes have to each othero and at what Diílanc.e froni
US they tneet together ; ic is to this Diftance that tvt
refer our Senfation, that is, to the fame Place where tb
ObjetStis. Wherefore if at any Time we are deceivej
in the Judgement we make of the Diftance of any Ob.
jed when we look upon it with both Eyes, it is be-
caufe we do not know exadbly at what Diftance the
Optkal Axes meet.

wd ' know^- ufeof but one Eye, wecM

íh/ Difiance know the Dijlauce of an Objedl j provided we move
ofan ohjeci. from One Place to another j for we have fome kind oí

o

ly. Another

Memory of the Pofition of the Optkal Axis in the,
Station, when we really attend to the Pofition q 0 í
another Station ; fo that we imagine two Opticih’' kx&¡
though there be indeed but one, and by that ineans
at the Diftance where they meet ; and to this we
reí^the Objeft.

16. AThird itS^Since w^annQjjnclin^be Optkal Axes to eac
l^e^D°¡fíañ 7 e in ^cery fffSimi^*Srüruér to make them meet
if an objefí. at one of an Objeft which is at a certain Di-

ftance from US, but at the fame Time we muft put
each Eye into a particular Difpofition or Figure, nece:
fary to fee diftinftly at that Diftance j we may prefunic
that Nature has fo ordered the Mufcles of the Eyes,
that they neceílarily procure both thefe EfFefts at the
fame Time : And that this is fo, we íhall have no Doubt,
if we obferve, that they who fee but with one Eye,
move, their Eyes in the fame manner to look upon Ob-
jefts at difFerenc Diftances, as they who fee with bot
Eyes. So that it is íufEcient, if our Eye be fo flatten
éd or lengthened in a particular manner by the Aftioii
/, of the Mufcles, as to caufe fome Alteration in the Braini

which puts the Soul upon conceiving the Pofition of di^
Optkal Axes ; And fince the perceiving this Difpofition
is the moft natural Argument to make us know the DI
ftance of an Objeft, ic follows, that the lengthning
fiattening the Eye js alón? ftifEcient to difeover thii
Diftance.

17. Bpt

'jiap. 32 » Natural Philosophy.

ij. 6at becaufe the Alteration of the Shape of one
ye only, when we make ufe of it to fee diftindiy at
fferent DiftanceSj is not fo fenfiblcj as the Alteration of
le Situation or Pofition of the two Eyes, when in
rder to look at diíFerent Diftances we turn thetn dif-
irentlyj that we may make the two Optkal Axis meet
ithefame Pointj therefore we are not to think, that
lis latter Alteration is fo exadly made, when it is deter-
lined by the otherj as if it were caufed by that Atten-
on which we have when we look with both Eyes up-
n the fame Point of an Objeft. And this is the Rea-
ra why we are more apt to be deceived in the Judge-
lent we make of Dijiance, when we uíé but one Eye
lanwhen we ufe both. Andindeed if we try to touch • an
Ibjeít at three or four Foot diftance, with the End of a
lick of about the farde Lengthj we ílrall findj that if
K look at it but with one Eye, we fhall mifs touching
two or three times together ^ whereas if we look at it
fith both Eyes, we fhall touch it the firft Time,
il^hatever the Alteration be, which is made in the
yesl^n we look upon Objeds at unequal Diftances,
is certain, that That Alteration cannot be at all fenfi-
)le, when the Diftanceis fuch, that the neareft Objed f
great Way ofF j wherefore we muft be very liable
nore deceived in our J udgem e nt of great Diftancqg^an
if fmall.

19. Befides the two forementioned MétSli^ jndging
)f the Diftance of Objeds, which are the principal ones,
liere is yet íome others : As Firft. Having often obfer-
ed, that an Objed appears more confufed the further it
I diftant from us, we make this a Rule of determining
le Diftances of Bodies, fo that according as they appear
nore or lefs confufed do we imagine them to be at a
jeater or lefs Diftance.

20. So likewife, becaufe we have often obferved,
bt an Objed looks of a brighter Colour, the further it

removed from us ; therefore when we fee an Objed
a brighter Colour than it ufes to appear of when it is
that it is at a great Diftance from

253

1 7 . That it ii
eajier to be
deceived in
the Jftdge-
menttee make
of the Dif-'
tance of an
Objeéi, when
xce looli lipón
it with but
one Eye, than
when we look
tipon it with
both Eyes,

lear; we conclude,
is.

\ 8 . it is
eaJie^oT us
to be\ieceived
in £ir fiidge.-
^nt of great
Difiances
than offmalU .

19. That the
Dijimélnefs
or Cortfiifed-
nefs of the /-
magcs nf Ob'
je¿ls, help US
in jfidging of
their Dfi
tance»

10. The fawi
Thing alfo
foUows from
tbdr being
more or lefs
bright.

K

I. jdn OhjeÓl at three or Four Foot
^'flanee) s to be obferved, that
íhe Srick muñ not be chruft direftly
Jpon the Objeít, but moved ob-
^uely, in the'íáme manner, as if,
^’ÍJeua Ringís cwrned Side-ways to

the Eye, we wonld try to run a
Stick through it i as is jiiftly re- '
•marked by Malbranch in bis £»-
qtilry after Trnth, "Boob I, Chap. ix
Se¿l. 3,

21. The

254

ROHAULT^s System

Par

/

|i. Thatwe 21. The Sicuatioti is another Means ílill of know
ji”Tce by^the Diílance of Objeds. Fon of thofe Things wh
sitnatkn al- wc imagine to be lower than our Eye, wejudge them
be farcheít.diftanc which affect the Eye winh the high
Rays ; and on the ocher Hand, of thofe Things wh¡
we imagine to be higher than our Eye, we judge the
to be fartheft diftanc which affed; the Eye with the lo
eft Rays.

^2. Furthetj the Interpofition of a great many oít
a greac maúy Objeds between us and the Object we loolc at, malí
US thinkj that the Diftance is greater than otherwi
7 hink,“ihat WC fliould becaufc the Diílance which we concá
íhe objeB is to be bctwixt cvcry one of them, is the Meafure whii
"¿iftan'e‘’‘" compute the Diílance of the Gbjecl by ; Tti
in the Inílance of the Moom when it is at the I.

. ^ above the Horizon, and we look at it through ti
Air only in which there are no other viíible ObjeS
we imagine it to be nearer to us, than when it rifes
Jéis ,• becaufe at thofe Times, there are a great mai
intermediate Objeóls upon the Earth, betw^
and it.

23. When we know the Situation and Diílance
Objecl, byjoining thefe together, we form a Judgi

i\ent of the Bignefs of it ; For, becaufe we imagine tt
cnities o f ah Qbje d, to be contained between tw
ílreigñttLin^^Vsí^gii'íSS^íIiye, which diverge froi
each ocMsKn - proporción to their Diílance ; therefoi
we eaíily conceive what.the Bignefs of the Óbjeíl is
a given Diílance. So that if at any Time we are d
ceived in our Judgement of the Bigneís of any Objeí
it is becaufe we are firíl deceived in our Notion
its Diftance. Thus, becaufe we cannot truly compre
hend che Diílance of the Moon or Sun from us, there
fore no Imaginación can reprefent thofe Bodies to us
great as they really are.

24. And chis is fo true, that the Stars feem to
fomewhat larger, when the Interpofition of vifible Ob

mhen they are jecls which are betwcen them and us, heips us to iiaa
Diílance to be greater,- For it is not o«
ing I to the Interpofition of Vapoura, ás the Ancieit

thougbi

f0777e t

the "Bignep*
Objc^s,

14. Why the
Stars feem
hijLger to US

1. To íhe Interpofition of Va-
ponrs, 8¿c.) Since che Angle under
■which the Moon appears when in
che Horizon, is not greater than
ordinary, it is evidenc? that nothing
©ughc here co be aícribed co che

Refraícion of tfee Vapours.
that chis Angle is not greater din
ordinary, isclearfrom hencej di
though every parcícujar Part of d
Horizon (as well the Diftances f
the Sears from each other as cbeSui
tbem

liap. 32* o/ N atural Philosophy. ¿54

lought, thac the Stars appear of difFerent BigneíTes,

¡f the Rays which carne from the Extretnities of them
the Eye of the Spedtator, were by that means re-
tafted, fo as that he fhould fee them under a bigger
Ingle. Fot modern A[lronomers who have meafured the
Ingles under which the Stars appeared, when they were
n the Horizon, and when they were at their greateíl
tkitudein the fame Day, ' have always found them the
ame.

27. It is to be obferved alfo, that very luminous or ís.'ihaiytry
iright Objeds muft needs appear bigger than they would
to if they were not fo bright. For if the Image which 'an they
ley imprefs upon the Bottom of the Eye, affedts not ¡o ¡lo.
mly a certain Number of Capillaments, but ípreads it
’ to the Extremities of other Capillaments which are
ibout it, it is the fame as if it had covered them
fo; becaufe the Rays have fo great a Forcé that alr^
thefe Capillaments are moved by them, and not at all
dred by the Motion of thofe Rays which come frona
»ther furrounding Bodies which affedl the fame
Paft?%but are very faint j therefore a bright Body ap-
lears fo much the bigger, as it tabes up part of the
)bje¿t which is not fo bright, whofe Rays are
:d up by it.

26. We may add i till fnrther; that th e Impreji^ of 26. my tht
very luminous Bod)^ífflpli*ÍPIPI!||as to^Etend it
felf%ll round to fome Capillaments, wMirtro Rays at Zt‘thrZgí a
'1 come to from the luminous Body ,• in which Cafe, Teiefaipc, ap-
is manifeft, thac che Objeít muft appear much bigger
lan it would do, if its Light were more faint. And her übjeéls
is certain, that we fee the fíxed Stars in this man- mag-
iier; becaufe if we weaken their Aftion, by artificial- '
ly contracfting che Pupil, and looking at them through
a Hole made in a Card with a Needle, they
appear much lels. But that which moft furprizes thofe

who

-'T

^hemíelves j my the Stars, when
they feem to be larger, feem alio
to take up moré of the Space which
krounds them,-) though, I fay,
every Pare of the Horizonfeems to
be equafly inlarged ,• yet the whole
Circle caonot contain any more
than qóoDegreesj wherefore Bo.-
dies in the Horizon are not feen
lindera greater Angle, but every De-
gree in the Horizon feems greater
than in the Meridian.

T. Haipe alíoays found them the
fame) Nay, they have found che Di-
ameter of the Moon, when at the
highelf, a little bigger, than when
íhe rifes or fets, See Malbrancb* s
Search after Truth, ’Book I, Chtif, ix»
Scól. 3.

2. They appear much lejs) Nay,
that the fixed Stars, .by reaí()n of
their immenle Diftance, are but ]ike
Pointsonly, except thac their Light
is a little diiated by Refraílion, is
evidttfic

256' ROHAULT^s System Partí,

who íée not the Reafoa of chis, is, that when we ]ool¡
at the Sears with a Telefeope, they appear as much di-
miniílied as other Objeds appear inlarged by itj andfoi
tbis fole Reafon, becaufe hereby the Forcé of their Rays
is very much weakned,

í-¡.Thtinon- 2.J. Ic is ccrtain alfoj that as the Knowledge of
ing thcBigr (.{jg Diñance helps us to find out the Bignefs, fo like-
jes, hei¡s us wife the Kuowing or the oigneís helps us to conceivs
^chitt the Diftance- Thus, when we know that a Man ii
about five or fix Foot high, when we fee him to
appear but very little, we conclude him to be at a great
Diftance.

Mufcles ; as it is certain, that the Images which are tben
impreíTed on the two Eyes, do not fall upon the Sjmp-
thetick Nerves, ñor reunicc in the Brain, fo we cannot faE
to fee the Objed double.

3T. Wmther 31. So likewifcj if 'wc look vcry intencly upon a par-
tojee an ticular Objed, and at the íame time fcother Objed be
'■ placed nearer or further off, which confequently cannot
imprefs its Image on che Sympathetick Capillamesits of the
two Optkk Nerves • in this cafe it muft impreís two I-
mages on that Part of the Brain which is the immediate

evident from henee, that when they
are abouc to be ecUpred by the
Moon i when they encerintoitsBo-
dy, iheir Light does noc decreafe
gradually (as that of che Planees does)
buc vaniíhes all at once, and at the

Endof theEcHpíés íc appears agala
allac once.

2. 3 «£ one Xmage upon tht
"Brain) See the Notis on xxXÍ*
uin. 20.

Qrg0

Chap. 32- of Natural PniLOsoPHr,' 257

Organ of Vifiorij and therefore ^ it muíl be feen
double.

32. Having feen how we come to l^now the Sitúa- 32.ffi11.wi
tion, Diñance, Magnitude, and Numberof Objedts by ífrceheMc-’
our Sightj nothing more remains but to examine how
we know whether they be ¡n Motion or at Refi. Now it
is not diíHcult to conceive, that we know a Body to be
in Motion ; firíl, when its Image appears fucceíEvely
applied to difiFerent Images of certain Objedtsj which
we do not compare with any other, but imagine to be
iramoveable; or when we find that we muft turn our
Head or our Eyes in order to have the Objed always
at the End of the Line along which we carry our
principal Attention,- or lañiy, when^ if we move nei-
ther our Eyes ñor our Head, we find it is gone out of
that Line. The contrary to all which makes an Ob-
jed appear to us to be at reñ.

ít mnft he feen douhle) It may
b^||:ther obferved faerca that if
the^^eít now mencíoned, be pía-
Cfd beyond che Poinc where the Op-
tical Axes meec, it will
Tab. Vil. then appear double in
Fig,2, fuch a manner, that of
che cwo Images, that

which is on che righc.áj|

feen wich che right Eye, ana fiw
on che lefc Hand wich the lefc
Eye j but if che Objeñ be on chis
Side Chat Poinc, chcn the Image
which is on che right Hand will
be feen with the lefc Eye j and che
Image on che lefc Hand wich che
righc Eye. The Reafon of W'hich
is, becaufe in the former Cafe che

Objeít impreífes íes Image on .
HZK che Jefe Side of the righc ^
Eye, and therefore is feen by it .
on che righc Hand, and on che EFG,^
the righc Side of the, lefc Ey/,
and therefore is feen by ic o!; che
lefc Hand i In che Jatcer^ Oafe ic
impreífes ics Image on the >

che ri^l'Eye, and
cherelor?T^|afs co^’ on che Lefc
Hand i and^^j^LE the lefc Side
of che lefc Eye, and therefore ap-
pears co it on che right Hand.

IVhat fnrprix.ing Thingsfúluxo frovz
thís Obfervatimi may be feen in
the Notes m the folh'xmg Chap~
ter.

^ ^

^

CHAP.

*58

ROHJULT*s System

Part I

C H A P. XXXIII.
0/ DIOP^RICKS.

uThatmr TN order to prove the Truth of fome of thofe Sup>
^vifimmaybl pofiúoHS wÉich wc have madc about Vifion ; wc
confirmed by ought DOW to confidcr, whethcr or no all thofe Things,
*’’‘tf’“'¡fdr Suppolitions ought to come to pafs,

fíremSortT when WC loofc through diflferent Sorts of Perfpedive-
efPerfpeSive Glaííes or upoii Loolcing-Glafles, be agreeable to Experi-
encej for this wül be a great Proof of the Truth of
Glajfet.^ ■ámíz Suppofitions.

2. We will begin with Perfpeñive-Glaíles, and firfl:
^ Z'^ded'when confidcr that Sort called Multiplying-Glajfes-, luch

'T ‘háidatthrdi^í.^ that io the Figure ABCD. Now it is evident in tl^
firfl; Placcj that without .this Glaís, the Eye E would^|^
Tab.' VIII, the Objed by means of the Rays which come irotn
í io G ; and becaufe the Superficies BC is here parallel
to the Superficies AD, whieh is oppofite to it, and there-
fore tHií^^efradion which the Rays fuffer when they
enter intoShe Glaí^||i|g||&||^^l||j|l|^e Refradion made
at their conifa2 oi^l^tiollows^nat the Eye ought not-
withílanding, to receive the ImpreCSon of the Objed in
. the fame Place G, where it would have received it if
therc had been no Glafs j and for this Reafon it ought
ftill to fee the Objed in F. It is alfo certain, that
the Objed F, would malee an Impreflion upon an Eye
placed in N by the Rays which it would fend thither,
if there were no Glafs between; but becaufe thefe Rays
now meet with the Superficies AB„ by which they are
fo refraded, that when they come out of the Glafs,
they enter into the Pupil of the Eye E, and afterwards
go on in fuch a Manner, as to fall upon that Part of
the Bottom of the Eye marked I, where they imprefs
É fuch an Image as an Objed placed in M would do ;

^ therefore this caufes che Eye at the fame Time that it

fees the Objed F in its true Place, to fee it alio in M.
So likewife the Rays which would excite Vifion in the
Eye, if it were placed in O, and no Perfpedive-Glafs
intervened, being in this Cafe refraded by the Super-
ficies CD, fo as to imprefs an Image of che Objed F
on the Part of the Eye marked H, where an Objed

placed

Chap . 3S - Natural Philosophy. 259

placed in L would make íes Impreffion if there were
no Glafs; it follows^ thac the Eye E ought to fee yet
another Objeít F in L. In a Word, it is eafy to infer,
that the Eye muft fee the Objed F in all thofe Pla-
ces, where the ftreight Lines terminate, which coming
from the Pupil, pafs through the feveral Sides of the
Glals, by which the Rays of the Objed are fo re-'
fraded as afterwards to make an Impreffion of it upon
the Retina.

3. I havenothing further to add to this, but only that •i-Whyit
fometimes the Objed when looked at through the Sides

AB, CD, may appear differently coloured from what it id.
does when looked át through the Side BC ; the Reafon
of which isj becaufe the Rays which come from the Ob-
jed through the Sides AB, CD, are refraded pretty
much in the fame manner, as they are by a Frifm,
has been explained before.

4. Let US now examine a convex Glafs fuch as that ^

jn the Figure CDEF. Now it is to be obferved, that
ILjt is the Property of this Glafs to colled into a Point differmt
fflyRays which fall parallel upon itj fo is it the Pro-

perty of it, to colled into a Point, likewife the feveAl thW
Rays that fall upon it from any fingle Point of an Ob- «
jed, with this Condition, that the Point where they are ^ x,
reuniíed is fo mucí^^^fiirthg^ifent froiy. the Glafs,
as the Point from*TOHPfn^Kayf\Darad is nearer to
it; and this latter Point may be fo n^, that the Rays
which proceed from it, may never be reunired at all,
but become parallel or fomewhat diverging when they
come out.

5. This being fuppoíéd, if the Objed AB be at a pro-
per Diílance from the Glafs, all the Rays which come majmüte tht
from every Point of this Objed, may be reunited again imagctfthe
in as many other Points. For inftance, the Rays which

come from the Point A may be colleded togecher in H, '
and thofe which come from the Point B, may be col-
leded together in G. Now if the Eye were placed in
the Point I, it is certain, that becaufe the Rays which
convey the Image to it from every Point are converging,
that is, enter into the Eye with a Tendency to unite toge-
ther ; therefore I fay it muft neceflárily be, fince the Re-
fradions of the three Humours of the Eye are made in
the ufual manner, that by means hereof thefe Rays muft
unite together fomewhat nearer to the Chryftalline Hu-
mour than they would otherwife have done/ Wherefore

S s if

How a
convex Glafs*

\

i6o RO H AU L'í’ s System Partí.

6. Hovp ít
makes oíd
Mtn fce more
difiiníl.

A

¡rf'X' ‘‘

if this Eye be the Eye of a young Man, which cannot
ñateen it felf beyond what is requifice to fee Objedts di-
ftindly, whoíé Rays fall upon it as it were paralkl j it is e-
vident, that fuch a Perfon will fee Objeds lo much the
more confufedly as the Rays which fall on the Eye have
a greater Tendency to uniré together more on this Side
the 'Retina . .

6. But if it be the Eye of an OU Man, which by the
common Decay of Age is become flatter than the Eyes
of other Men ; becaufe the Reafon of fuch a Perfon’s
leeing Objeds confufedly isj that the Rays which come
from any Point in an Objedb are not reunited when they
come at the Retina, which they fall upon Iboner than
they íhould doj therefore a Convex-Glafs makes them
fee cii[linBly j for it makes the Rays more converging, and
¿¿helps the Humours of the Eye to reunite them juft
when they come at che Retina.

7 - The Difia7ice of an Objeít looked at through fuch

'tnake! anOb- ^ ought to Zapear greater, becaufe the Difpofitio

(

Dijiance,

agr/Iu7‘‘‘ of the Rays which come from any Point is fuchj^S
caufes the Eye to put it felf into luch a Figureí^as
bccañons the Mind to imagine the Diftance greater.
And this is the Realon ‘ why we think the Objedi;
to be further ofE if we be not prejudiced before-hand
in our really is.

8. As

1 . W')y roe thinl' the Objeéí to he
furjher off) Here che famous Dr. 'Bar-
roxo propofes a very greac Difficulcy
in his Optical Ledttres, viz. the i8.
to'.rards che End* Horoever, íays he»
I will not leave off-, till I have propo-
fed tbyon a very greac DIfficnlty (oaf
nf the Stmerity I urve tn yoii^ a7id to
Trnth^ by no means to be dijjembled)
vohich is C(¡?itYadi¿iory to that Opinión
íohich I have heen recommending to
yoxi, at leajl cannot be fulved by it,
It is briejly this. Lee the Point

be etspo^cd to the Leus
Tab. X. CDliFy at fjich a Di-
Jiance» that the Rays may
be Jo bent as tn tend tarvards nnitin^

' fomexv/fcre in the Alxis HD, and Itt
the Point PI be the Place vehere they
wect, OT the Imageof the Point A as
voe have all alo7ig befare ajfcrted, viz
the Focus ; betrveen this Point and the.
Glafs let its íuppofe the Eye to be
any vphere placed, lasb.^ in rohat
Place ought the Foint A io appear

to be, In the Nature of Thingt it
canntt be feen behind at the Foint
H {becanfe every Tmprejjlon that af-
feéls the Senje, comes from the oppo-
fite Pííríj viz. A) andit is contrary to
Experience alfo. Novo it feems to fol-
íopoi from the Doéirine voe have laii
dovon^ that it Jlmild appear to be
befare «íj and at the greatejl Dia
Jiance pofftble (a Dijiance excecáing
any that roe can imagine), For the
lefs diverging the Rays that come
from any Objeói arCi fo mnch the fnr-.
ther dijiant do voe conceive it to be
[if voe be not prejndiced conctrning
its Dijiance before-hand',] and that
Objeói which fends forth parallel Rays
we imagine to be the mofi dijiant
that can he, In Reafon therefore^
one wortld think, that when the Rays
come from the Objeñ converging.^ it
Jhíuld (ippfar, if it were poffibíe^ at
a greater Dijiance yet. Bnt in this
Cafe it may be asked in general,
rohat is it that determines the ap-
^arent

Cha - p . 33 - Natural Philosophy, 261

8. As to the Situation, that will appear the fame as ufu- s. Why ¡t
alj and the fame as if we look at the Objedt without ’¡eB“ppear'’fn
the Glafs, becaufe the Eye fees the right Side of the its trae SitU'-

Objeít

farent place of the Point and
makes it appear Jornetimes nearerj
and fomctimes further ojf^ and al-
ioays in the fame Proportion. To
vohUh Scrupíe roe can give no An-
fvoerfrom the xAnahgy of any Thing
that has been hitherto faidi only that
the Point A ought alroays to appear
to be at the greatejl Dijiance. "But
Experience ¡hotos the contrary» viz.
that it appears at dijjerent Diflan-
ceS) accordmg to the dijferent Pojition
of the Eye betvoeen the Points F and
H, and fcarce ever [if at all) at a
fíirther Dijiance than the Point ^
reaUy is j but many times it appears
much nearer, nayt the more the
Rays which come to the Eye cow-
tke nearer the Image of the
O^S^pproaches. Thtts, f the Eye
be^^^in the Point V» the Point
A voitl feem to be very nearly in its
trne Place j if the Eye be moved
bacifward to the Image will feem
to approach nearer y and it will ap~
ptar fill nearer df the Eye be in lor Lt
andfoby degrees till the Eye
fomewhere near íf, whcre t'tj'c 'vojcá''"
will appear very neati and begin to
vaniJJj confufedlyj All which feem
to contradiü ozsr ^rgnmcnts and 0-
pinionS) or at leaji^ do not very well
agree with them. And this Expe-
riment not only contradiéis onr Pío-
í/ow, but all Qther that I l:now of
eqnally. It fcems fo much to over-
throw that antient and common one»
which is more a-l'in to oms than
any oxher^ that the learned Tacquet
wasforced byit to renounce that Prm-
tiple (fípon which alone^ almiji all
bis Catopiricks depend) as uncertain,
andnotto be depended upon, where-
by he overthrew his own Do-

Uriñe In the prefent Cafe there

is fomethirig that lies deep hid in the
Snbtlety ofNaturet which perhaps can-
not be difcoveredj till we underjland
the Nature of Vijion more perfeélly,
Concerrdng which ^ 1 confefst I have
not yct been able to thinh of any
Thing to flatter my felf withi much
lefs to give my felf entire Satisfa-
¿lion. I therefre leave this Dif-
fcfilty with yoKi (indvpifh yoabeiter

Succefs in falving it. Thus far the
faraoüs Dr. Barrow.

And indeed it mufi: be acknow-
Jedged, that rhere is a very greac
Dimculcy hete. For it is evidenci
chaca Candlcj the Rayscoming from
which, are coileóted togecher, and
made co converge by a convex Glaísj
hüwever near» we, by afurprízing
Miftake in our Judgemenc, conceive
icto be, does nocwithdanding af-
teft the Eye when it is placed in í
or L, exaftly in che fame manner,
as it would do, if thofe very
carne indeed from an infinite
íhnce, as will appear by che follow-
ing Obfervacions.

Firjif If the Lens be ib broad,
thac we can fee the Candle through
ic with both Eyes atthe fume time,
though we endeavour all we can co
niake our Optical Axis diverge to a „
diftanc View, yec che Candle will
never appear fingle, bu: always
doubíei in fich a manner double,
thac of the cwo Images of che Can-
Hand one will ap-
■'^péar"'üti 'Uíí^'i^ght Hand? and che
lefe H-and oñ^;?n che iefe Hand.
Whenceicis moft manifeíf, that the
Place from whence we ought co judge
che Kays come, is beyond thac where
the optical Axes meec, be icac never
fo greac a Diflance ; that is, che
Candle will afFeíl the Eye in ihe
fame manner as if ic were at an in-
finite Diñance. See ihe Notes on
Chap. xxxii. Aru g i .

Neither can ic be faid here, thac
che Candle is nottherefore feen dou-
ble, becauíe ic is feen, asic were, ac
an infinite Diftance j but that it is
only an accidental Thing, and effe£f-
ed by the Inrerpoficlon of the Glafs,
For íf we look through a concave
Glafs, ic does not appear double j
and ic may be feen Tingle through a
Convex* GlaíSj if eicher the Eye or
che Candle, be fo near che Glafs, thac
the Rays fall upon che Eye, noc
converging, but only lefs diverging j
in which Cafe: fuch Glafies are of
greac Ufe co render the Sighc more
diftinét.

S 3 Secondlyt

\

z 62

RO H A UL'T’s Systiím Partí,

Objeft B, by means of the Ray VI, which ¡s on the right
Hand of the Ray SI, by means of which, it fees the left
Side A.

Secondiy, The Reaíbn of the Ap-
pearance of a Candle in chis man-
ner when looked ac chrough a con-
vex Glaís, is exaftly the íáme, as
thac of a Candle feen ere£t when
the Rays are refleíted by a concave
Looking- Glaís. In both Cafes the
Rays are convereing j in both Ca
fes the Objeít leems equally near.
Now in a concave Glais, if when
the Image is feen ere6t behind
che Glafsj a Sdck or a long Reed
be fü puc becween che Candle and
the Superficies of che Glaís as to
íi^nd perpendicular to che Glaísj
tlfe Image of* thac í'tick ought to
appear of an infinite Length be-
hind the Glafs (as Tacqmtt has de-
monñrated in his Catoptricks,’Book III
JProp, 22, and as the Thing ic
felfíhows us)5 and yec che Image
of che Candle muft necefiarily ap-
v.pear beyond the Image of chis
Stick i however near therefore we,
chrough Prejudice. judge the Image
of the Candle to be when alone, ic
is yec evidenc, thac ic does really
affeft the Eye, as if chere a
Hice diñance between.^^ld ihelame
muft be íaid of a coa^x Glafs.

Now here is the greac Difficulcy
(as the learned Perfon before-men-
lioned obferved) how it comes to
país, Chat when the Rays fall up-
en the Eye as if they carne really
from an infinite Diftance, yec the
Candle does noc feem (asoné would
expe£t) to be as remóte as poíli-
ble, buc always very near, though
íbmecimes nearer than other, and
thac in a cercainand conjftant Pro-
porción.

• Now having confidered chis Diffi-
culty on all Sides, I at laft found ouc
the following Solution of thisíurpri-
2ing Pb^nomenon.

. Becaufe we cannoc judge of
the Di flanee of che Candie by the
meeting of the optical Axis (for
in this Caié, thoíe Axis can never
meec ar al] at the Candle, as was
beforc demon/lraced 3) and becaufe
the Judgement which we make of

the Diftance of Objedts by one Eys
only, is always the woríl and moft
uncertain, and becaufe the true Di-
ftance of the Candle is known be-
fore j therefore from Prejudice and
PrepoíTeífion, it muft always feem
to Qt pretty near to as» To which
we may add, thac we cannoc by our
Sight perceive any Diftance» how
greac íbever itbe, if there be no-
thing in the intermediare Space:
Thus the Body of che Sun, though
we very well know, thac ic is at
an immenfé Diftance from us, yec
ic feems very near 5 and were ic
noc thac we imagine to our felves,
from che Concavicy of che Heavens,
a cercain Radius of a Sphere, we
ftiould chink ic ftill much nearer.
Thus if we look at the
chrough a very long Tube,
hinders our feeing any oth« Bo-
dies, it feems to be ac the End of
che Tube.

Seco 7 idlyi ic ought alfo to appear
fometimes nearer than other^ and
that in a ttrtain and conjiant Rropor-
when che Eye is placed
neSf the Glaís, asinV, the Candle
feems hircher off (as by che Laws
of Opcicks ic ought to do) than ic
does wichouc the Glaís 5 now if che
Eye be removed backward gradual-
Jy, the common Refra£lion of the
Rays wili be fuch, thac che Candle
muft neceflarily feem larger and
brigheer, in che fame Proporción as
che Eye recedes from che Giafs. Now
chis Largenefs and Brigheneís is the
Rcaíon why ic feems nearer and
almoft cloíe to the Eye.

And this is confirmed from henee,
thac if che Rays of the Candle are
firft crarifmicced chrough a concave
Glafs (thac the Bignefs and Bright-
neís of ic may be diminiíhed) and
then by pafling chrough a convex
Glaís they be made to converge (as
when we look chrough an inverced
Telefcope of two Glaífes) then we
eafily imagine the Candle to be ac
a very greac, and almoft infinite
Diftance*

5. But

Chap. 33* «/ N ATURAL Philosophy. 26 $

9. But this Objedí" will appear fimewhat higger, becauíe 9-

the Raya VL SL as they enter hito the Eye^, are indi- *
ned to each other with a larger Angle, than they are be- H^ir.
fore they were refradled by the Glafs^ fo that they
feeming to come from the Places 2 and 3, imprefs
an Image of the Objedl upon the Eye as big as if they
poíleffed all the Space between 2 and 3.

10. If the Eye be placed in L, the Rays which come lo.Howit
to it from any Point are ftill more converging j and
therefore if the Sight were confufed before, it will be f,ii¡h¡¡ler‘'''
much more fo now. And becaufe the Rays XL, and TL, and m<¡re ««.
which come from the two Points A and B of the Objed,/®/*^^^
roake a ílill greater Angle than SL VL they muft make

the Objedt appear yet bigger. Whence it fliould feem

to follow, that the Vifion fhould not be fo clear, but

more obfcure , becaufe the Rays which imprefs the Im^e

of the Objedt on the Eye taking up a larger Space up-

on the Retina, each Capillament of the Optick Nerve

receive fewer of them in Proportion : However it is ’

ti^ainj that we can then fee as clearly as if the Image of

t^lObjed: were fmaller. For thereare a greater Num-

ber of Rays, which come from every Point and are

difpofed by the Glafs to reunite, that enter into the

Pupil when it is fo placed as to fee the Objed very

large, than when it is placed where the Objed appears

fmaller. . ^

11. So likewife if the Eye be placed 'In Y, the Objed n.Honh
ought to appear very bright and clear, becaufe all the

Rays which come from any Point of the Objedj and Miy cmf^.
fall upon the whole Superficies of the Glafs do then en- fii-
ter into the Pupil, but it muft, notwithftanding this,
appear very confufed, becaufe the Rays being alréády col-
leded together when they are about to enter into the
Eye, 1 are refraded afterwards by the feveral Humours
of it, and are by that means difperfed again ,• fo that
thofe which come from the fame Point of the Ob-
jed, impreís an Image on a great many of the Capil-
lamenta of the optick Neryes, upon which the Rays
which come from other Neighbouring Points impreís
íheir Image alfo, and this makes the Image qf the Objed
wholly (onfufed,

I, .Are lefraSed aftermaTílj'j Are I theBottomof tha Eye.

^ilperfed ag^in when [hay come ac >

§ 4 -U-

254

ROHAULT’s Syste

M

Part I.

r

>

II. mu it 12. If the Eye be placed in M, the Objeót muft ne-
wTappMr ceíTarily appear invertedi for ,we fee the lefc Side A by
invfrtelínd mcans of the Ray HM which is on the right Side of
conjufíd. GMj by which we fee the right Side of the Objed.

It muft alfo neceílarily appear confufedy as well becaufe
the Rays which come from any Point as A, cannot be
exaóHy colleded together at all beyond the Glafs, fo
that the Eye cannot put it felf into any Figure which
will reunice all the Rays that come from H ,• as be-
caufe when the Rays really come from H as from
one Point only, they fall fo diverging upon the Eye,
that it cannot lengthen it felf enough to reunite them
upon the Retina. The Firft of thefe Two Reafons
íhows US, that in this Cafe it is impoflible for the Eye
to judge what Diftance the Objed is át ; and i that it
■ Ahe ms in that Place in which we before-hand imagine it
to be.

13 - If

I. Jt fee)7JS in that Place^)

Here we meet wich another Diffi-
.culty, concerning the Place in which
the Image oughc ro appear, almoft
as great as che former, which Mr.
Vechales propoíes in this Mañner,
!Boot II. Prop, 1 1 . o/ hi s Dtoptrid’s>
There ist fays hca «j/wni
Pifficiilty iñ expíahifjf^' the Manne^
kavo the Eye fees the Place of the
Objeéi, but in this Cafe
Tab. X. there is a very particular
Di^culty, becaufe Reafon
and Experience do not feem to agree
together} nay, the Experience here is
contrary to other Experimehts a/fi.
For it is ev'ident jrom Experience,
that the Objcél A'B is not feen in
fhe Place of its Imagti viz. in GJH}
vahen the Eye is placed in M- for I
have tried That a htmdred Times.^
find tiirned the Glajfes all Ways in
prder to jirtd if 1 could pojjibly malte
it fticcced fo. Horce-t,er, according to
jReaj'or;^ it onght vahhout all Doubt fo
be feen in the Place of the Imase^ viz.
in GTH. For when the Objeéi kA’B
afeéis the Eye hy the Ray- of its
Image-, it Jhould feem as ij it oitght
fo to -rfeéi the Eye as if it roeré
fn GTH. For ij the Pchit j'or
Infante, wcre in fi, it xoculd fená
forsh Rays from H to the Eye in M»
and though ¡t he in its proper Place •
yjz^ in the Point yef it ferfs

forth Rays in the fame Mar^’j^as
if they carne from the Point'
therefore it feems as if it Jhould
affeéi the Eye in the (ame Manmr
as if it mere in the Point H.

To this Difficulty, this famous
Perfon anfwers, That the Body AB
.t-really feen by the Eye
in the Place of its Image GYHj but
becaufe ic can be feen only byone
Eye at a Time, therefore by a mi-
ítaken Judgemenc, we imagine ic to
be furcher from us. Thnsfar He.

I have oftencimes fo ordered the
Glafs, that che Objeiít AB (which
oüght to. be a Candle) may be feen
wich both Eyes N and P at tlie
íáme Time. If it be a very large
Glals the Candle may very eaíily be
íeen wjth boch Eyes at the' wme
Time.

Having therefore made exaífi Ob-
fervacion of this Matrer chrough fuch
a Glafs, I aSirmí that the Btfdy AB
is feen by the Eyes NP exactly in che
Place oí its Image GYH ;

For if che optical Axes Tab. X.
be ib direéfed, as to ineet
m the Superficies of the Glaís, che
Candle wlll always be íeen double,
and in fuch a Manncr double, that
the right Hand Image is feen by the
lefc Eye, and the lefc Hantl Image
by the right Eye. Whencéic is moíl
manifefí, that the Iinage is p)ace4
witj^i^

Chap. 33 * Natural Philosophy. 265

13. If the Éye be fuppofed in N, the Second oí-' thefe 13. fíen the
Reafons will not take Placea and therefore the Objedt

üught to be feen a little more diflinñ, but always in- tUmdTnd
vertedi for the Reaíbn above-mentioned. And as to icfs confufed.
the Bignefs of it, we judge of that by the Largenefs of
the Angle made by the Rays which come from the Ex-
tremicies of the Objeít, at their Entrance into the Eye»
cüinpared with the Diftance which we imagine it to be at.

But ic mufl: not here be omitted, that the S paces OP
and QR, through which the Rays which come from each
Extremity of the Objeft diffufe themfelves, is fo much
the greater as ic is further diftant from Y, where rhe
Rays which come from every Point of the Objedt meet.

And this maltes the Space QP, where the Eye receives

the ImpreíEon of Jthe two Extremities A and B at the

fatne Time, to be fo much the bigger alfo 3 fo that there^^

is a large Space for the Eye to move abouc in, where it ' A ^

will always fee the whole Objeót. , -

14. Hicherto we fuppofed the Objech to be fo far re- »4. Howft
íiií|^ from the convex Glafs,. that the Rays coming

froffilfc: might eaíily be reunited in the Bottom of the
Eye; lee us now fuppofe ic fo near the Glafs, that the
Rays which come from any one Point of it, have no
Tendeney towards uniting together, after they are paífed
through it, but are only ipade much lefs diverging than
they were beforec 'fíí^uic-anOi -íhe Eye to be at

fuch a Diftance the Glafs, that the Refraótions which
are made at the Entrance into each of the Humours be
fuch, as will caufe the Rays which come from any fin-
gle Point of the Objedt, to unite again in one Point up- ■

■ on the Retina; in this Cafe ic is evident, that the Vifion
muft be exceedingly diftinéí. For, befides that the Rays
which come from difterent Points of che Objeít, do not
at all confound each other, the whole Image impreífed
by them is fo large, that there is a fufficient Number
of Capillaments of the Optieb Nerve, to caufe the Soul
to perceive 3 great many Particulars, which ic would

wichin the Place of Concourfe of che
optieal Axesj that is, becween che
Glafs and che Eye, -x'/x. in GYH, See
the ÍTotes on Chap.XKXii. Art. 31.
Bul further, if che opcicalAxes be
|o direfted as 10 meer on this Side
che Glafs, che Candle will be feen
fingle, and manifeílly on chis Side
ihe Glafs.

Buc in the former Cafe, where che
optieal Axes were dlrefled to a
Point fur.her diílanc, becaufe che
Image of a Candle does not cerini-.
nace che Sighc like a íóUd Body,
and becauíé wewere beforehand pre-
jiidiced concerning che eme Place yf
ic, therefore ic íeems to be at a greaCf
er Diftance.

otheiv

266

ROH AULT's SvsTEM

Partí

'íy, Con-
eerning Mi-
er^copes^

>

otherwife have taken no Nodce of, if the ímage had beeii
fo fmall, that the Rays which carne from two adjoin.
ing Points of the Obje& had been forced to meec to
gether in two difFerent Points of one and the fame Ca-
pillament.

ij. Upon this Foundation it is, that thofe fmal
GlaÁfes which we cali Mkrofiopes are made. They con
fifi of I one Glafs only^ which is fo convex, that if i
Flea, or any ocher fmall Objedt be placed at about
an Inch Diñance from the Eye, and the Glafs be put
between them, it will caufe the Rays which come, from
any fingle Point of fuch a fmall Objeít, and which di
verge very much, to diverge afterwards fo little, that
the ordinary Refradions of the Humours of the Eye,
will determine them to unite in one Point on the Retm.
■■ ■ E v this Means the Eye which without a Glafs cannot
íee any Objed diftindly which is nearer than a Foot
Diftance from it, may be made to fee one which is
twelve Times nearer ir. From whence it follows, that
the Diameter of the Image which this Objed im^l
upon the Retina is twelve times larger^ and coní^uent
ly, that the whole Superficies is a Hundred aud For-
ty Four times as large^ as it would be, if the Oh'
jed were at a Foot 'Diftance ; wherefore fince it eX'
tends it felf up on a Hu ndred and Forty Four times a
many Capillanj^iílf!" ihf tHS^OputS: ^rve as it wouIí
otherwife do, the Objed cannot biPbe feen very
ftindly.

id. Let US now examine a concave Glafs, fuch as
‘r’ñoas^hf’ Figure CDEFGH, the Property of which

'^Étyfwhkh iS’ according to what was before faid, to make the
Rays which it receives from any fingle Point of aa
Objed, to become more diverging than they were be-
fore they paíTed through the Glaft. Thus the Ra
which come from the Point A, and fall upon th
Pare of the Glafs marked VX, fpread themfelves aftet
they are pafléd through ir, from R to Zy and thofe
which come from the Point B, and fall upon the fame
Space VX, extend themfelves through the Space YT
Further, it is alfo the Property of a concave Glafs, fo
to incline the Rays, which come from two diflerent

How ít

come from
different
Soints of an
Qbjeíi»

Tab. XI.

T. They con fifi of one Glafs only.)
Thefe are fome which coníiíí: ot fe-
veral Glaíles, that are miich more
Bíce. Wliac, and how furpruing

Things have been found out B]
the Help of chele MUrofeopeSi mayb
íeen in Mr. Hopk^s Micrographjf aoi
in ochers.

PníníS

;hap. 33* o/ N atural Philosoph y.

’oints of che Objeft, to each orherj that when thcy
neet together, they make a lefs Angle than they would
lo, if they had not paíTed through fuch a Glafs. For
nftance, the Ray MI, which comes from the Extremity
)f the Objeófc A, and the Ray LI, which comes from
he other Extremity B, make fo fmall an Angle, viz.

MIL, that they feem to come from the Places mark-
EdN, O.

17. Whence it follows, that if the Eye be placed in I, , 17. íto»
ind look upon the Objedt AB, it will íée it confitjédlji :

Bccaufe the Rays which come from every Point, are fo fafed.
divgrging, that the Refradtions of the Humours of the
Eye cannot make them unite in fo many Points upon
the Retina.

18. However, there may be fome Eyes fo much long- 18. nat
er and more gibbous than ordinary, as to reunite the¿^^/^*'*
Rays which they receive from any üngle Point of a

ftant Objeft, before they come to the Retina, fo úm. ftiJiSiy.
they can fee only near Objeds diftinótly,- They there-
for($^o have fuch Sort of Eyes as thefe, may make
good^fe of a concave Glaís to fee diftant Objeéts
diftindly with ; becaufe by this Means the Rays which
come from any fingle Point of thé Objed are made fo
diverging, that the large Refradions made by the Hu-
mours of fuch Eyes, do not reunite them before they
come at the

19. If an Eye of the ordinary Figure be placed at a 19. Ko»
greater Diftance from the Glals, as at P, it will fee fime-

what more éftinBly, becaufe the Rays which fall upon *!Z‘s¡ghtiefi
the Pupil from any ñngle Point of the Objed are lels andfimtinut
diverging than they were in I ; and on the other Hand,
an Eye too long or too gibbous will fee it fo much the
more confufedly as the Point P is further from the Glafs,
becauíé the Rays which come from any fingle Point of
the Objed, being lefs diverging, the Refradions made in
the Eye, determine them to meet before they come to
the Retina.

20. But whatfoever the Figure of our Eyes be, whe- 20. That h

ther they are fitted to fee Objeds that are near, or fuch hí-

as are at a Diftance j whoever makes ufe of fuch a •

Glafs will fee the Objed in its true Situation- for the

Rays which caufe us to fee the right Side of the Ob-
jei^:, come to us from the right Sidej and thofe which
caufe US to fee thq lefc Side, come from che left Side.

ai. As

i68

RO H AU LT* s System Part I.

ix^That it
makes it ap-
$ear nearer

ío US,

2i. That ie
makes it ap~
pear iefs.

25. That it
makes it look
t^nally clear*

>

24.. That it
makes a large
S^ace for the
Ohje ¿2 to be
feen in.

2f. Concern-
ir/g Telefcopes

21. As to the Difiance, it makes that feem lefs than
it really is, becaufc when the Rays which come frotn
any one Point, enter into the Humours of the Eye,
they diverge juft as much as they would dO:, if they
did indeed come from a Point of an ObjeS: much
nearer.

22. And as to the Bignefs; becaufe the Extremities
of the Objedt are feen by Rays which make a lefs Angle
than they would make without a Glafs, it follows, that it
muft appear much kfs.

23. Becaufe the Rays which come from any Point
of the Object are made more diverging by paffing
through a concave Glafs, it follows, that fewer of-
them can enter into the Pupil, than if they had not
paffed through the Glafs; however the Vilion ought
rjpt to be the lefs clear upon chis Account ; becaufe
"tnis is made good by the Image being impreíTed on
lefs Space of che Retina^ fo that every Capillament
of the Optick Nerve is; fufficiently Ihaked to caufe
US, when we look through fuch a Glafs, to 1 ^
Objed as clear as when we look on it witKCüt a
Glafs.

24. To what has been hicherto faid concerning the
concave Glafs, we may add, that the Space RT, which
contains the Rays that come from the two E.xtreini-
ties of the OWed, “Tein^^er^Arge, it follows, that
the Eye may lee the Objed entire in any Part of chis
large Space.

2^. One of the beft Inventions of our Age, is that of
Telefcopes. For by the Help of chem we have not oa
ly difeovered fome Particulars in the Sears, which were
not obferved before, buc they íhow us alfo a Multicude
of new Stars in the Heavens, which we cannot lie
without them, ñor íhould we ever have come to the
Knowledge of them otherwife. They were indeed firfl:
difeovered by Chance,- buc the Invención appeared fo
furpri2.ing, and fo ufeful, that the greatefl: Genius’s have
laboured hard to bring chem to the higheft Perfedion
polEble. I cannot therefore forbear explaining the Na-
ture of chem in chis Place ; and che fo doing will very
much confirm all that has been hirherto faid about Vi-
íion. They confift commonly of two Glaífes, fixed to
each End of a Tube; That Glafs which is at che Eud
next the Objed, and is for that Reafon called the Oh-
jeíi Glafs, is a little convex, and the other Glafs which

i5

Chap. 33 * (>/ Natural Philosophy. 26 ^

¡sat the End of» the Tube nexc the Eye, and is therefore
called the Eye-Glafh ¡s on the other Hand, very ^ con-
cave, that is, much thinner iii the Middle, than at the
extreme Parts.

26. The Objeñ-Glafs caufes all the Rays which come í 6 .TheTre-
from every fingle Point of the Objed, tounitetogether ve-

ry nearly in as many differenc Points, on a Superficies °
which we are to fuppofe on tbis Side the Glafs, at a
greater or leís Diftance from it, according as the Glaís
is more or lefs convex ; now becaufe the Rays which
come from difFerent Points pf the Objed, crofs one ano-
ther as they país through the Glafs, it is eafy to con-
ceive, that they paint íiich a Sort of an Image upon this
Superficies as we have before íhown they do upon the
Retiaa, and that it is fo much the larger, as the reuniting
of the Rays caufes it to be at a greater Diftance fron|.„ -j.
the Glafs : If therefore the Bottom of the Eye were puf ■'
in the Place of this Superficies, and it were poffible for
the Humours of it not to make any Refradions 5 we
l]idsi|W have a very large Image imprefled on the Retina,
by l'^ns of this fingle Glafs, and it would fall upon fo
great a Number of the fmall Capillaments of the Op-
tickNerve, which would receive diftindly, the Impreffi-
on of every fmall Part of the Objed, that it would
be impoffible but that the Vifion muft be very di-
ftind, ' "

27. But becaufe the Humours of the Eye cannot be ^ 7 -
hindred from cauling the ufual Refradions, they tnañ
neceíTarily fo refrad the Rays which come from every
Point of the Objed, and which had before a Tendency

to unite together, that they will unite before they come
at the Retina, and then feparating again, will imprefs a
confufed Image upon that Tunick. Now the Eye- G/;íy?
is fo fitly placed between the OhjeSí-Glafs and the Place
where it would make the Rays meet j that it caufes thole
which come from any Point of the Objed converging,
to become parallel, or rathera little diverging^ but yet
it does not hinder the Rays which come from diff'erent
Points, from being as much difperfed as they were when
they croffed each other in paífing through the Objedt-
Glafs. And thus the Refradions neceíTarily made by the

I. Concave) There are alfo Te- I ing which, fee Rc¡^¡s Phyjicks, "Bnok
Icicopes conGftirig of cwo, three, I VIH. Tari. II. CAdy. xxxi.'s, xl, xli.
w four convex Glafles ; Concern- I

Hu-

27Í

ROHJULr>s System

Partí

tíumours of the Eye, inftead of being injurious, as tlie¡
were without this Glaís, become very uíeful wich ic ; foi
íhey unite thofe Rays which this Eye-GIafs difperfed
and by this Means the Iinage which the Objed impret
fes on the Reíma becomes perfedly diftind, and at tht
fame Time very large. Whence it foUows, that tli(
Objed is feen diílindiy and » fo much the bigger as the
Rays which come from any one of thefe Points, are
lefs diverging, and make us think it at a greater Di
flanee.

ít.n^ythefc 28. The befl Curvature that can be of the Superli.
Ghffes, the cies of GlaíTes for Telefeopes, is, ^ that of an Hjperk
'^e! make the ^ny fuch like Figure, and not the Curvature of

Stghtfe mnch Sphere. But Workmen have not yet been able to make

ihe more ob-

t. So mtich the higger as the Rays
tohich come fro7n any one of thefe Points
are lefs diverging^ and make us think
it ata greater Diftance.)

Thac ísa by how much the Rays
of every Pencil being lefs difperfed,
make ic appear further off. For
the further che Objedl feems to be
from usj the more do we neceflári-
3 y imagine che Pencüsof Rays, which
croís' one anocher as chey país thro*
the Objed Glaís, to divaricace, th 3 ^
is, the Objedt íeems fo much the
bigger.

2,, That of an Hyperbolat or any
fuch like Figurcy &c.) Caries tooka
greac deal of Pains abouc thefe fort
of Figures, and abouc the manner
of poliíhing GlaíTes, bur with no
greac Succefs. For ic is evidenr,
thac Spharical Glaíles, as they can
be more eaíily and more accuracely
made, than Elliptical or Hyperboli-
cal ones; ib are they co be prefer-
red before fuch upon chis Account,
becauíe they do more exadtly re-
fraít che Pencils of Rays which are
out of che Axis of the G!aG. And
indeed, ic is not to be aferibed
to the Unficnefs of the Figures
of the GlaíTes, but to quice other
Caufes, that Telefeopes cannoc be
nade abíblutely perfeíi and com-
pleac. The Two Principal of which
Caufes are thefe.

Firpi The unequal Refrañion
che Rays chemfelves i {See the
on Chap. xxvii. ^rt, yz.) by wliich
means neither che Eye-Gh7(^0j\iú
is Convex) can be made t^opheres
fmall enongh to magnify the Ob-
jeft 3 ñor che Objeól-Glafs of a fuf-
fícienc Aperiure, co render ebe Ob.
je£t brighe and diílindb, but every
Thing will ímmediately be

wiíh_ Colours, and confounded by

tü^íi'áequal Refraftion of che Rays.
For ihe eminenc Sír Ifaac Neatm
has fliown, thac che Difference be*
cween che Refradtion of the
and moñ refrangible Rays, is about
the Twenty feventh Pare of tbe
wholc Refradllon of the mean re-
frangible Rayss and that che Fotus
of the moít refrangible Rays is
nearer to the Objeéi-Glafs than che
Focus of che leaíl refrangible ones
by abouc a Twenty feventh Pare
and a Half of che whole Diíhnce
between the Objeél-Glafs and the
Focus of the mean refrangible Rays.
(Opc. p. 74.) And therefore the
greateíl Errours which arife feom
tbe Jpharical Figure of the Glafst are
very much lefs than the Errours
which arifé from the unequal Re-
fradtion of the Rays chemfelves)
nay, in fome Cafes, the Propordan
is as greac between thera, as 1200
to I (pag. 8p.) From whence i:

abun-

^ap. 33* 0 / Natural Philosophy.

heir Glafles of any other Curvature but that of a
Sphercj of which chey take fo fmall a Part, that it does
not fenfibly differ from an Hyperbola. But ihen there
is this Inconvenience attends it, that there does not fall
fo many Rays üpon it from any one Point of the Objeft,
there would do if the Glafs were larger ; and confe-
quently all the Rays which come from the whole Objeft,
and which fpread themfelves upon a large Portion of the
letina, íhake but a very few of the Capillaments of the
Oftick Nerve ; and this is the Raafon why we fee Things
more obfcurely, than when we do not ufe fuch a Glafs ;
and the longer fuch Glafs is, and the fewer the Rays are
which come upon the Pupil from any Point of the Ob-
jeít, fo much the weaker and more obfcure muft that
Objeft appear.

abundantly appears, that not the
(fhicñcal Figure of the GUJfeSs but
ihe djfferent Refíaogibility of the
Rays themfelvesj is the Cauíé why
Telf(§^pes have not hitherto been
madt'^pblutely perfefc and com-
pleat) and that there can be no Re-
medy for this InconTenience by any
way figuring or poHíhing refraéling
Glajfes’i this excellent Períon, ac
lengcb ínventedj and agreeaWe to
Experiments, propofed the manner
of making a Telefcope which ."CSld
caufe the Obje£l: to be íéen by Re-
ftxmr. Concerning che Conftruñi-
on and Ufe of which Inílrument»
See Opcic. pag. 95.

Secondiy, Jf the Theory of making
Telefiopes could at Ungth be fully
IroHght into FraSicet yet there ‘vould
it certain Roundsy beyond which Te-
Itfcopes cotild mt perform, For the
Air through which we look upon the
ÍMr/j is in a perpetual Tremor 5
M may be feen by the tremuloits Mo^
im of Shadows ca/l from high
Towers^ and by the tvoinkling of the
fiíced Stars, Rut thefe Stars do not
tmnkíe when viened through Telef-

copes which have larger ZAperturesl
For the Rays of Light which pafs
through diverfe Parts of the Aper-
ture tremble each of them apart,
and by means of their various, and
fometimes cantrary TremorSi fall at
one and the fame Time upon
different Points in the ^ottom of the
Eye, and their trembling Motions
are too qukk and confujed to be per-
ceived feverally. And all thefe illn-
minated Points conflitute one broad
'lucid Pointi compofed of thofe many
trembling Points confitfedly and in-
fenfibly mixed with one another by
very Jhort and fwift Tremors, and
thereby caufe the Star to appear broad-
er than itis, and withont any Trem-
bling of the Whole. Long Telefcopes
may caufe Objeéls to appear brighter
and larger than Jhort ones can do»
but thty cannot be fo formed as to
take away that Confnjton of the Rays
which arifes from the Tremors of
the Atmcfphere. The only Remedy is
a moji ferene and ^niet Air^ fuch as
may perhaps he found on the Tops of
the higheji Mojintains above the grof-
fer Clofíds, Newt. Opticks, p. 98.

C H A P.

270

372 RO H AULT^ s System íartl

C H A P. XXXIV.

Of Looking-Glajfes.

I. o/fíf ¿rf- fJESIDES plain Looking-Glaílés, which are every
ftrt„c Sms where ufed, there are two other Sorts, viz. Convex
%h¡°es”^~ and Concave ones, not to mention thofe which are com-
pounded of thefe three SorcSj which are capable of being
infinitely diyerfify’d,

■í.Thtctm- 2. Each Sdrc of Lüoking-Glafles has indeéd its par-
mm Propirty ticulat Pi'operty or Manner of reprefenting the Objeíij
°ímkinZ“ agreC:, thac they fo refled the Rays

Light, that che Angle of Imiáence is equal to the Au.
gle of Reflexión, and that che refleded Ray is not in the
leaft turned afide;, either to the right Hand or to the Left;
thac is to fay, i the incident and refleded Rays atvl-
ways in the íiime Plañe which is perpendicular ^the
Superficies of the Glafs j whence it follows, that though
the vifible Objed íends forth from every Point a Multi-
tude of Rays which are refieded by the whole Superfi-
cies of the Glafs, yet a determínate Number of them
only can come to the Eye^hws jt is fixed in a certain
place.

r. The incident and rejleüed, Uays
are al-mays in the fame Tlane which
is, .perpendicular to the Superficies of
the Glafs,) This Property wonder-
fully perplexed the famous Dr. "Bar-
roa ,• you xoUl not eafily find any.
¿ood and citar Account of this Mat-
ter amon^fi the H'riters of Opticks'j
almofi every Thing that they alledge-,
roith relation to it, is either begging^
the firfi ^Principie, or elfe Ubonrs un-
der fióme inccmprehenfible Ohficurity\
ñor do 1 much voonder that this Jhonld
be the Cufie of thofe xcho altpays con-
fider a Ray of Light as one conünued
fireight Line ; xohich if granted, I
can ficarte believe it pofjible to afifign
any good Reafion for this Thing. /
therefore thtnk thai' a Ray of Ligkt
is not a mere^ Line^ bu: a hody endued
xoith all the Dimenfions', fio that it
may be cylindrical or prifmatical, &c,.
Sed. ri. Buc iheie do noc

feem to be any neceíBcy of recurrirg
to ihe Figure of che Rays 5 it is ail
one whecher they be cy-
lindrtcal or prirmatical, Tab. 11.
whether they be fblid Bo> Fig. 6.
díes or indivifible Lines.

For lee GBL be che Superficies of
the Earth (which 1 fuppoíé to be
plain and ímooth) A the Norchi I
theSouch, ABaRayofLight. Now
it ís evident, that chis Ray of Lighc
is carried with a double Determina-
ción, the one AG downwards to
the Earth, the other AH direñljr
to che South i the firfi: Determina-
ción is refifted t)y che Superficies üf
the Earth, the other isnots che Ray
therefore oughc to go on diretfiiy ro
che South with chis Decerminatíon,
Chat is, in .a Plañe perpendÍcu!arto
che Superficies of che Earth i ñor can
it curn cowards che Eafi in an ob-
ligue Plañe.

5 . This

Giiap. 34 * Natural PniLosoPHy, 273

3. This being fuppofed, lee AB be a plain Looking- fíom a
Glafsj by Means of which the Eye C fees the Objeót

DE; having drawn from any Poinc at Pleafure, íup- makesiny
pofe D, the Line DIL perpendicular to the Superficies «

of the Glafs, we Ihall fhow that this Poinr D ought to
be feen in the Point L of this Perpendicular^ fo that the Tjb. vu.
Diftance IL, which we imagine it to be at behind the 3 ’
Glafs, íliall be equal to the Line ID ; ‘ for it is eafy
to demonftrate, that the Rays DF, DG, by which the
Point D afFedl the Senfe, are fo refiedted in the Lines
FC, GH, that they enter into the Pupil CH, as if
they really carne from the Point L : fo that this di-
verging of the Rays caufes the Eye to put it felf
into fuch a Shape, as gives occafion to the' Soul to ima-
gine that it fees the Objeft really in the Point L.

4. And as the Point D was taken at pleafure, what 4. Thattht

has been íaid concerning that, ought equally to be urA
derílood of ali other Points of the Obje£l ; and tberefore it plar'as'j—
is evidenr, that when we loofc upon an Objed in a teyondaphm
plain Looking-Glafs, the nuhole Image ought to appear .

behind the Glafs^ as the Objeói is placed before pUu/on\i,is

it, Siiie of it»

5. It is further evident, that this Objed ought alfo to s- That a
appear of the fame Bignefs, as if it were really placed in LM :

For the Space which the Image feems to take up, is com- 'ctf^htumake
prehended between twq parallel Lines which are at úíq objeaap-
lame diftance from e'ach other as the Extremities of the r

übjed are.

Laftly, This Objed ought fo to appear in the Look- 6 . That u
ing-Glafs, that the upper Pare fhould be feen above, and
the right Side on the right Side, and fo of the reft.

Thus the Part D, which is higher than E being feen by
the Rays of Incidence DF, DG, and by the refleóted
Rays FC, GH, which feem to come from the Point
L; and the lower Part E being feen by the Rays of
Incidence EN, EO, and by the reflededRaysNC, OH;

I . For it is fa(y to áemonfíraie^^c.')
For che Angle DFI ^ co che An-
gie CFB i and the An-
Tab. VII. gíeCFB:^ tu thb An-
l'ig. 3. gle IFL1 chereíore rhe
Angle DFX to the
Angle Il'Lj and the Angles at I

T

are right, and che Side IF common,
Therefore che Triangles DFI and
IFL are fimilar and equal. In ]ike
manner che Triangles DGI and
IGL are fimliar and equal , Tnere-
fore the Triangles DGF and FGD
are fimilar and equal. Q_ E. D.

which

274

RO M A U LT^s System Fart I*

which feem to come from the Point M ; we refer the
Senfatioii which we have of the Point D to the Place L,
and that which we have of the Point E to the Place M,
which is lower than L.
j-Thjtitií 7. What has been faid concerning one Eye, ought
Thm^whe underftood of the ochen And indeed if

th"r^m 7 iaak wc fuppofe the Spedtator principally attentive to look
upantheGLifs upon the Point L, le will ealily appear, that his two Op-
“r‘tuhbHh Ayes, will be fo inclined to each ocher, that they
' will feem to meec in che Point L. Whence it follows,
that the Rays which come from every Point of the Ob-
jecl to enter into one of the Eyes, leem to come from
the fame Points beyond the Glaísj from whence the Rays
feem to come which caufe evéry Point of the Objed; to
be feen by che other Eye.

s. Thata g. As to a convex Lookbíg-Glajs, fuch as that in the
figure reprefenced by ABCj by Means of which the Eye
D fees the Objeít EFj i it is eafy to apprehendj that it
ihe ohjfíiap- Pj refleóts the Ráys which fall upon it from any Point of
^D'ijhmce íí- the Objed, fuch as EB, EG, that the refledted R^s
hindíhe BD, Gti diverge juft as much as if they really gí^e
Giafsythattit Point I, which is at a much lefs Diftance'be-

*Tab/ix. bind the Glafs than the Objeít is before it; And this
Fíg- n is the Reafon why we fee che Image much nearer thaa
when we look upon a plain Looking-Glafs.

9. That tí p Further, the Point L fEom«'..hence the Rays MD,
“¡farfmlTer. feem to comc, by which we fee the Point F, ^ is

fo near the Point I, chat IL appears much lefs than EF,
that is, a convex Looldng-Glals makes che Objed; ap-
pear much lefs than it really is.

10. Thát it 10. But thoLigh in this a convex and pláin Looking-
p^n^Hits’’^ Glafs differ from each ocher, yec they agree in another
tn.l ‘óuulii. Particular, viz. that they both malee the Objed; to be

feen m iPs trae Situation, as appeSrs from henee, that

I. Ir is eafy to apprehend» &c.)
This may eafily be demonítrafed, if
we draw a ífraight
Tab. IX. Line BG repreíenting
Fig. I. a plain Looklng-Glaísj
and compare ii (as to
the Áuuañon ) wiih che Tangents of
che Poines B and G.

2. 7 .Í fo near the Point T) There
are two Reafonsof chis. Firít, Be-
taúfe che Image in chis Glaísj by

reaíbn the Rays of every Pencíl
are mure difperfed, is noc fo far
diftanc from che Vereex of the
xAngle of Vijion asín a plain Look-
ing-Glaís, becondiy, Becaufe chis
Angle of Vijion is therefore íeli»
becaufe che POrúon of the Glafs upon
which che Rays that áre reflefted
co che Eye, falJ, is leís chan in a
plain Lüoking-Glafs.

the

Chap. 34- Natural Philosophy. 2.75.

the Rays EBD, EGH, by which ihe Eye fees the
Point E are higher than the Rays FMD, FNH, by
which it fees the Point Fj which is the lower Part
of it.

II. As to Vifion made in looking upon a concave '*

Looking-Glafs, it may be diverfify’d feveral Ways ac- *

cording as the Eye and the ObjeCt are in differént Po- mahs the
fitions. Let us fuppofe a concave fpherical Looking-
Glafsj whofe Cencer is about the Point Tj and let usgTJatlr “ di-
imagine in the firfl: Place, that by Means thereof the Eye
Dfees the Objedl: EF which is pretty near thé Superfi- ‘bcfVí it.
cies of it. This being fiippofed, the Rays EB, EG which Tab. ix.
come frotn the Point E, are fo refledled to the Pupil,
that BD, GK diverge but very little, and feem to come
from the Point H, which is at a much greater diftance
bhyond the Glafs, than the Objed is on this Side of it. "

And this malíes u» refer the Image of it to a greater
diftance than if we look on a plain Looking-Glaísi and
toa ñill greater than when we look on a convex Look-
irjfcGlaís.

jR. As to the Rays which come ffom difFerent Points iz. Hoaít
of the Objed, they are in this Cale fo refleded, that
thofe which aíFed the Senfe from the upper Part of the p 7 Jfia°thÍ~
Objed, are higher than thofe which aíFed the Senfe sítaati-
from the lower Part.pf it ; thus the Rays BD, GK, °u"lrthlif*
which caufe the Senfation of the Point E, are higher than thfobjeT.
the Rays ID, LK, which caufe the Senfation of the
Point F j and thefe Rays ID, LK, feeming when they
ehter into the Pupil as if they carne from the Point M,
are the Caufe of feeing the Point F as it it were in M.

And becaufe HM is much bigger than EF, it foK
lows that the Objed ought not only to appear in its true
SitiiatioK-y but alfo Tfiüch bigger than it really is.

15. The Rays EN, FO, as they go towards the Glafs Jj-
divide more and more from each other ,• where.ftire
they be continued backwards, they muft meet together UmnZT"'''
fomewhere in the Point P, and afterwards dividing a-
gain that which was uppermoft, will be lowcrmoft, and
that which was lowermoft will be uppermoft ^ whence
We cannot but conclude, that if an Objed be in QR,

I. And this waVes sis refer the\ che concave Lookíng-Glaís here is
Imuge, &c.} See the Notes on Chap. 1 the fanae as that of the convex Glafs
Sxxiü. Art. 7 . fot me Cafe of| there,

T 2

ic

j/}.. H’íin» it
niaj he that
the Pupil oniy
(itn he jeen» ■

376 ROHAULT’s System Paftl.

it muft appear znverted ^ but becaufe the Rays which
ought to affeót the Senfe from any íingle Point of it»
fall in fuch a manner upon the Superficies of the Glafs
that as they are refleded to the Eye» they crofs one
another in íeveral Places between the Glafs and the Eye i
and fo cannot be re-united in one Point upon the Retina»
therefore the Vifion muft be very confuíed.

14. If the Eye be placed exadly in the Center of a
concave Looking-Glafs» it can fee nothing but the Pupilj
for thofe Rays only which fall perpendiculaiiy on the
fpherical Superficies» are refleded to the Center j and
thofe Rays only which come from the Center fall per-
pendicularly. upon the Superficies j wherefore the Rays
which go from the Pupil and fall upon the whole Su-
perficies of the Glafs» return from thence to the Eye
»*irtigain, v/hich muft therefore fee the Pupil fpread all over
the Glafs.

if. Fow the ly. If the Objed EF continúes in its Place, and the
Eye be moved to X» between the RaysBD» GK» pro-
longed i it is evident» that it will ftill fee the Poin^
by means of fome of thofe Rays which it faw it by^í^-
fore; but it will not fee the Point F, by Means of the
Rays ID, LK» which carne to it from the Part IL of
the Looking-Glafs ; inñead-of which» thofe which fall
from F upon Y» and go from thence to X» will make
the Point F to be feen» and conTeíJuently it will feem
to be foraewhere in Z» and fo the Objed will appear as
largeasHZ.

16. If the Eye continúes in D, and the Objed EF be
removed backward to P, the Rays which come from
eyery Point of it» and fall upon any Part of the Glafs
as BG» will be lefs diverging thin they were before.
Wherefore after Reflexión they will become converging,
and more difpofed to unite when they enter into the
Eye, than they ordinary are» and fo muft really unite
before they come at the Retina, which will make the
Viílon coiifufed. But it will be ftill jnere conjufed if the
Eye be in that Place where the Rays which come from
cv'cry Point of the Objed meet together again j for thefe
Rays at their Entrance into the Eye will begin to be » fe-

Ohjeci ynny
appear very
large.

Tab. IX,

t6» Hoto it
7Hay appear
ab[utntely
confiíjed-

I. .Pc.iratcd hy Refr.inim) They ' merely by recedingfrom the Point
e Iipa.atcd, iioc by Ketraftion, buc ' where they crofs each other.

parated

€ ha . p . 34 ‘ Na^úral Philosídphy. 2,77

parated by Refradtioní and will be feparated more and
more by the Hamours of ir.

17. If the Objed remain in P, and the Eye be remo- t;. ^mthn
ved a litde from the Place whcFe the Rays which

come from every Point of the Objed reunite, the
Rays when they enter into the Pupii, will diverge too
much; whereíore becaufe the Eye cannot lengthen
ic felf enoughj the Objed will afpear cmfufed here
alfo.

18. But if the Eye be moved fo far backward from fíoKtht
that Place where the Rays reunite, that the Rays which

enter into it, be not too much diver^ng, the Viíion
ought then to be dijlmóí ,• and what is here very re- ‘mcave
markable, and the moft furprizing Effed of a concave G°hjíy'
Looking-GIaís, is this j that becaufe we are accuflomed
to refer our Senfation to the Place from whence the
Rays which affed the Eye from every Point of the
Objed leem to come, therefbre tíie Ivmge mufi afpear
between the Glafs and the Eye ; ib that if a drawn Sword
lltorefented before the Glaísj we íhall fee the Blade
come out from the Glals, and grow longer and longer
as we approach nearer to it ; becaule the Rays which
come from every Point df the Objed, the nearer ic
is, are che lefs inclined to each other aftcr Reflexión,
and therefore meec tngether at fo much the greater Di-
ftance. ’■

ip. Ic.

I. The PhffiHomena of a concave
Looking-Glaís, may be very pro*
perly reduced to five Cafés.

Firñ, Lee ¿Aí L^rrow or the Can-
ále EF be near the Glafs. Now
becaufe the Pencils
Tab. IX. EBGKn, FILKD do
Fig, 2. not crols each other,
whereíbever the Eye be
placeda whether ic be near or at a
diílance ; therefore the Image HM
ought always to appear ereét. And
becaufe the Rays of thofe Penciis
are refleéied, not converging to each
other, but only lefs diveiging, tíiere-
fore the Candie ought to appear to
be at a certain Díñance beyond the
Glafs.

Secondiy, Lee the Candle be in
the very Center T, Then becaufe
mli the Rays iall perpendicularly

upon the Glaís. they

muíl neceíTarily be all Tab. IX*

refledled to che Center Fig. 2.

i t felf 3 -therefore where-

ever che Eye is placed, ^ out of rhe

Center or any ot the Lines tending

to the Center, icü evident, thatic

cannot fee -the Candle at all i-n the

Glafs.

Thirdly, Lee the Eye be in the
Center T, Then becaufe no Rays
but thofe which fall perpendicular-
ly are refieñed to the Center :
therefore che Eye can fee nothing but
its owD Image l^ead all over the
Glafs.

Fourchiy, "L^t. the Cattdle^l^be
further dijiant from the Glafs, and
the Eye iQp further dijiant alfo,
Then becaufe che Penciis QP, RN*
crofs each other, h is evident» that
T 3 «he

278

19. That Oí-

je&s do not
pahit their I~
wagti on the
Superficies of
Zjooking-
Glajjes*

ROH AUL't’.s System . Part K

19. It may be obferved here, that they have been
very much miftaken, who have afSrmed, that •vifék
ObjeBs paiiit their Images upon the Superficies of Lookifig-
Glaffes; for every Thing there is ib confufed, that there
iS'ño one Part of the Glaís but receives Rays from all
Parts of the Object at the fame Time ; and indeed it is
certain that all Objeds which we fee by the Help of a
Looking-Glafs, do not imprefs their Image any where
elfe but on the Bottom of the Eye, unlefs when we fce
them by Mcans of a concave Looking-Glafs, under the
Circumftances mentioned in the foregoing Articles ; and
in that Cafe it is certain, that the Image impreffed by

the Image of the Candle oughc to
appear inverced co che
■Tab. IX. Lye KD. And bccaufe
Fig. z. the Kuys oí every Pen-
cil are reflctTccu con-
verging, and aher meeting fome-
where in a Focus, go from thence
diverging to the Eye i cherefore the
Image wiil not appear beyond the
Glafs, biu on chis Side of it, in
chut Focus. So likewiíc;, ¡n amihe'f
Tigarci bctaLife the Pencíls GD, BC
croís each other, it is
Tab.XVII. evidencj that che Image
Fjg. 5. of che Candle GB ought
10 appear inverced fo
the Eye in Qj, and alfo on chis
Side the ClaO:, and non beyond ici
becaufe che Kays of every Pencil
croís one another in a Focus, as was
befnre explained. Buc why in chis
Cafe we fhould noc imagine ic to be
very near, (unlefs we look ve'^y in-
lipón ir) when ic is rcally ve-
ry near, See the Notes m Chap. xxxüi.
Ji: 7 t, 12. for che Cafe is the fime
• herc as in the Perfpeíftiv.e Glafs
there.

Fifchly, Let the Candle Gli be nt
fotKe Difiamc from the Chfsy and
the Eye M very near it. Then
becauftí che Candle GB
Tab.XVII. is feen by other Pen-
3 ' cilsGHM, BCM which
do not crofs each other ;
it is manjfe/I, that rhe Image of GB
pughc to appear ereci again, but more
cpnfiifed. .

But ,¡n thisCaíe it is parclcularly
m Í?e pbíerved, that the Eye M hach
no way to jiidgü either in whac
Piace, or ac whijc Diíiaac.e behind

che Glafs the Image of
the Candle oughc Cu ap' Tab.XVII.
pearj for fince rhe Rays Fig. y.
of every Pencil con-
verge towards each other, thac is,
do not come from any given Point,
buc as ic were from an infinite Di*
flanee, to enter into the
fince thofe reflefted Rays
do noc meec with their refpeclive
Perpendicularsof Incidence D P, FL,
(from which meeting the Place of
che Image is always decermined)
there remains nothing to judge of
the Diflance of the Image by but
mefe Ptejudice.

Ic was very ill * Catropcicks
therefore in * Tac- BoDk-ilI.
qneti afeer he had fb Prop. 30.
well demonftrated
under this Head-, that the refieíledl.
TJiage in any Loohing-Glafs is alxcays
fefn in the? lace veherethe refieñedRays
meet with their Cathetus vj Incidemey
(the Cathetus of Inddencefs a Line
drawn from any Point in the ObjeS
perpendicular 10 che Glaís) 10 excepc
this lañ Cafe as contradiéiing this Ax-
iom; whereas ic is no ways conya-
diítory to ic. For when che Eye is
in fuch a Pofition, as to receive the
refleñed Ray.s before they meet with
their Cacheci of Incidence, the Image
cannoc be feen where they meec»
becaufe they don^c meec any where i
neicher is it feen in any other certain
Place; buc ic afFeifts che Eye as if ic
' carne from an infinite Diflance; in
the fame Manner as when the Rays
come converging out of a Perfpe-
élive-GIaf See the Notes on Chap»
xxxiü. Nrt, 7,

C/iap. 34 * o/'NaturalPhilosophy. 279

the Objeílj is noc upon the Superficies of the Glafs, but
in the Airj in the Place where we imagine we fee the
Objedj and where the Rays which come from every Pare
of itj are uniced afeer Reflexión. ^

I. Befides íiich Looking-Gla 0 es ’
where we look upon one SuperScies
only, we may alíb confider Perfte-
¿tive-GlaíTes, or cercain dear Glafiés,
as Looking-GlaíTes confiftingot cwo ^
Superficies 3 according co che Variety
ot which, tbere is alíb a wonderful
Variecy of refle£ted Images. For ncc
only the firñ Superficies which re-
ceives che incident Ráys out of Air,
but alio the fecond Superficies which
receives the Rays going out of Glaís
inroAir, exhibits a refíeíteJ Image,^
as may be feen by placlng a Candle
before fuch a Giafs.

Firñ then? lee a Candle be placed
before a Glaís which is plain on boch
Sides 5 then the Images refiedted by
eadf^^perficies, will boch be feen
creétrod exaftly like each ocherjcx-
cepting only, thac That which is re-
fiedled by the farther Superficies will
feem a litde more obfeure, becaufe a
greac raany of tne Rays have already.
been refleded by che firíl Super-
ficies.

Secondly, Let the Giafs be plain
on the one Side, and convex on ihe
other 3 thenif the Candle be placed
before the convex Superficies, thel-
mage v^iíl be reflecbed erefi: by each
Superficies (unlefs the Giafs be of
fuch a Thickneís, and che Fore-fide
of ic fo convex, ihac the Rays in
paíTmg chrougb ¡c are madeconverg-
ing, and afeer having been refleéled
by che plain Superficies, and paíling
a fecond Time through the convex
Side, meet in a Focus before they
come to the Eye 3 in which Cafe
the Image from the latter plain Su-
perficies will be feen ínverted) but
that which is from the firft and
Convex Superficiesj will appear lefs.

But if the Candle be placed before
che plain Superficies, chen the Image
refledled from the firíl Superficies
will be ere£fc again, and that from
chefurther Superficies, which is con-
cave within, will be refleñed in-
verted, and' will aTo feem co be
much nearer co the Eye, chan thac
from the firíl and plain Superfi-
cies,

Tbirdly, Let the Giafs be plain on
one Side, and concave on the ocher„
Then if che Candle be placed before
the concave Superficies, che Image
refleiíled from ihe firíl Superficies
will be Ínverted, and that from the
furtheroneí cre£l. But if the Can-
die be placed before the plain Su-
perficies, the Images refledled from
each Superficies will be eredl, but
thac from che further one, which is
convex within, will appear leís.

Fourrhly, Lee the Giafs be con-
cave on one Side, and convex on
the other. Then if the Candle be*
placed before the concave Superfi-
cies, the Images by each Superfi-
cies, will be inverced 5 but if before
che convex Side, they will be boch
ere£l.

Fifchly, Let the Giafs be convex
on boih Sides. Then che Image of
che Candle placed before íc, willal-
ways be refleSled ereól by the firíl
Superficies 3 and always invert by
che other Superficies, which ís con-
cave within,

Laílly, Let the Gláfs be concave
on both Sides. Then the Image of
the Candle placed before ic, will al-
ways be refle£led by che firíl Su-

erficies inverced, and always ereSl

y the latcec v/hich is convex
within.

X

T4

CHAP.

28o

RO H A UL'T’s S ystem Pare I,

C H A P. XXXV.

A Solulion of [orne Prohlems concerning Vifion.

I. 0 } the '^HOUGH I have been very large upon this Sub-
Kc’iL'fírt ^ Vifiorij yet I doubt noc buc that I bave

7tf¿urd‘"Ld paffed oyer a greac many curious Queftions, the Solu-
dooinmards tioH of which may pe'rhaps be fomewhat diíEcuk to
jnm a Can- who are not well aequainted with our manner of

Explicación. Thac this T*atife the'refore may be as
licde defeñive as poílible, and to fhow the Uíefulnefs
of it, I íhall here propofe fome of thefe Sort of Que-
ries; and leave the Excellency, at leaft the Trutb of
our Hypothelis to bejudged of, by feeing how ^a^it
is to reíbive chem. And Firft, I ask ; Whence il0is,
that duhen we look upon a lighted Candió at a little Bi-
Jlance rxith our Ejes ’u.'inking-, there feem to come Rají of
Eigbt from tloe Fíame of the Candle, atiá dart vpwards
and downwards into the Air l And inhence is it alfo,
that if an opake Body he put hetireen the Eje and the
Fla.ee <where lue fie the uppermofi Rajs-, use fiill cottlinue to
fiee theni, a7id on the contrary, ceafie to fie the ¡o<wermofi
fab-ix. In order to underfland t;he Reafon of thefe Phx-

r'g- 5 - notiiena, lee us confidcr the Eye A, the Eye-lids of
which Hj E áre fo near each other, thac there is only
a very narrow Paílage left, throiigh which the Rays
which come from the Candle BCD pafs to imprefs its
Image on che Pare of che Retina EFG in the Manner
above explained : Furcher., ic is to be obferved, that
the Parts FI and I (which are ufed to touch one ano-
íher when the Eye is cbfe fhut,) are fo fmooth, thac
they refemble f two fmall convex Looking-GlaíTes,
‘jvhich refledl che Rays of Lighc falling upon, them^ to-

Tmo fmttll corve X Lool'ing-
ijjíi') The Rays in chis Cafej are
^oc reflefted by ihe ínward. Super-
fcci'sof thp£ye-lids themfelves, in
■j^e manner oi, L^oUng-Glaílés^ buc

are refraíted by ihe Humour w^ich
flicks tü the out-fideof them ; iij
explainipg alhhe reít or this Phangr
menon> :hs Reafon ¡s the fame.

f

28 i

Chap. 35. cf Natural Philosophy.

wards the Retma, to che Parts of it EK, FLj whicTi
otberwire would noc have been afFeóled buc by Objeñs
which are about BM and CN. Wherefore the ImpreC-
fion made upon EK caufe the Appearance of bright
Rays, which we refer to the Place BM, and the Im-
preffion made on GL caufe the Appearance of the
Rays which We imagine to be in CN. But thac which
is rnofl: worthy of Obfervation here, is, that the Pare
of the Fíame B, which illuminates the lower Eye-lid I
by Rays which are reflected to the upper Pare of che
Retina LG, caufe the Appearance of the lower Rays
CN ,• wherefore if an opake Body OP be put between
the Eye arid upper Pare of the Fíame, we íhall ceafe
to fee the lower Rays, and continué to fee the upper
ones, becaufe they are feen by Means of the Rays
CH, which come from the Botcom of the Fíame, and
which are noc intercepted. And all the DifFerence thac
we íhall íind in thefe upper Rays, is this ; thac where-
as before they feemed to be in BM, they will now
to be on this Side the opake Bbdy OP. Buc
when the Eye is open as ufual, thac is, v/hen the Eye-
lids come no nearer than S and T, we oughc not to
fee thefe Rays of Light • becaufe the Rays which fall
upon thofe Places which we now compared to Looking-
Glaíles, enter but a little Way into the a^ueous Humour
3C furtheft, and are hindred frum going any further by the
Uveous Tunick.

1. Vi'hence is it that when a Fire-hrand is turned round, 2. ofaFín-
•we jée a Circle of Fire through which it paffedl The Rea- irandiumid
fon of this, is, becaufe the Fire-brand makes a circular
ImpreíBon upon the Retina, and the Motion of it being
very quick, lome of the Impreffion made at firll remains
till it returns again.

3. From this Ph^nomenon we may draw this Con- 3 . That tu
cluíion, that though Viílon is made in an Inílant, it does »/ J«-
however continué fome íhorc Space of Time.

4. Whence is it that a Cannon-Ball, or any other hlack ^.fVhywe
Body, pajjing very ¡ptúck befare a white Wall, cannot be
perceived at all > The Reafon is, becaufe a black Bo-

dy making no Impreffion upon the Eye ,• the Ball in- «ty
terrupts the Rays of Light refleded from the Wall, fo
very little, that the Motion which thefe Rays'excited
in the Eye juft before, is continued ¡n it for fo íhort a
Time.

5 . JFhjf

2 §2

Why foms
TgrfoJis can
fee Objeéis
¿I0inélly-i at
a certain Di-
jeance Qnly^

6^ OfT'ifíon
thró^ a Hole
made voith a
Si&tált*

RO H A U L T*s S Y s T E M Parí f,

5- Why do fams Perjom fee dtfmílly at a certain Di-
fa7ue only, a7id fee confufedly at a greater or leffer Di~
flanee ? Ic is t. becaufe they are fo accuftomed co look
ac that Diílance, that the Muícles by which tlie Figure
of the Eye is altered, are grown and uncapable
of performing their Office j in the íame Manner as the
other Mufcles of the Body are uncapable of moving
the Members of it, if they have not been exercifed
for a long Time. To which we may add ; that the
Tunicks which contain the three Humours of the Eye,
are fo hardened^ that they will not fo eafily yield as
before.

6. Whetice is it that an OhjeS which appears co7.fu-
fed, when we look at it too 7iear, may be feen very di-
flinBly at the fat/te Diflance through a Hole made with a
Needde Í7i a fl77e Card, or a Piece of Paper ? The Rea-
fon is, becaufe the Eye then receiving a lefs Quantity
of Rays from every Point of the Objeft, each of thcm
paints its Itnage but upon a very fmall Space, fo that
they which come from two neighbouring Points,
confound éach other’s Adtions. 2

7 . Whetice

1 . becaufe they are fo accujiom- '■
rífj &c.) This oteen happéns to fome
particular Sorr. of Workm?n, as En-
j;raversj ere. and ought to be look'd
upon as a particular Sorc of Diítem-
per.

2. It may alfo bere be enquired j
Why a very fmall opabe 'Eoáf Juf
pended in the Middle of an Hole be-
íween ihe Uye and a ^reat many
Z-i^htSi is mnltiplied as to be
feen before every Light? The Rea
ion isa becaufe tbe Rays crofs one
anoiher in that Hole^ and are in-
cercepted by the fmall opake Bo-
dy. í-ec US imagine

Tdb. VI. GHILN to be che Eye,

PEDFQ^ che fmali Hole
•j-n the Paper, HD the fmall opake
Body íufpended in the Middle of
rhe rfoJe,- and A, Bj C, three Can-
«lies. This being, íüppoíed, che Bo-
dy HD will , incercept tbe Ray
BO j theií the Shadow of that Bo-
dy wilt.-fall on Ó, and rherfíore
the Bpdy-ju* feif wHí be feen in B 3

ío likewiíe ic will intercept the are paífed beyond ic. In order to
Ray AX ; fo that its Shadow v.'iil have theíe Rays mect cogechef be-
’ fore

rail lipón X, and theref'ore ic will
be feen in A. Laíily, ic will al-
fo intercept the Ray.CY» whofe
Shadow will fall on Y, and there-
fore icwill be feen in C. Neicher
is ic neceíláry that an opake Body
fliould be fufpended in a Hole at
all : For fince che Rays that come
from a greac many lucid Bodies,
crofs one another ín tbe Tunua
Cornea^ íf you fix your Eyes upon ‘
a Pire oí burning Cpalsj and put
a very Hender Iron-rod cióle to your
Eye, i: will be greatly mulciplied,
and feen as ic were before every
Cual.

Secondiy, Why an Objeíi is ¡een
doTible when lanked at with one Eye
throKjjh troo Hales made in a Pa
per clofe to each other? In order
to accounc for chis ic is to

be obíérvedj that che Objefts are
never feen double, 'but when all
the Rays of the fame Pencil, meec
together before they come co the
Bóttom of che Eve, or after they

bhap. 35 * óf N ATUB.AL Philosophy. 283

7, Whsnce is it that thofe ixiho have heen coucijed for 7- why thev
Cataraéís, can Jee hut confufedly after-wards, and why do
'they nioant 'very large convex Glajfes in arder to fee di- Cataracu
R'mñly ? Before we refolve this Queílion, ic is to be ob- /"’z*
ferved, that a Cataraét is not a Pearly Subftance form-
ed between the Aqueous and Chryftalline Hucnours, as
has been long imagined, but is an Alteration made in
the Chryftalline Humour it felf, which has thereby in-
tirely loft its Tranfparency and is become opake, if not
through the whole Subftance of it, yet at leaft in lome
Part of it ; which may very eaüly be, for this Humour
is compofed of a great many Membranes one upori
another, which become vifible when it is boikd. Whence

fore they arrive ac the
tab* X. Botcom of the ILye, let us
fuppofe CDE to be the
Pupil ot a yoLing deep Eye, the mid-
dle Pare of which D is covered by
the Tmall Incerítice betv/een the
Holeíq^jjf the Paper ; and let OQNPR
be tb 3 teotcom of the Eye. Now
becaufe this opake Body intercepes a
gteat many of the Rays, and for
that Reafon mak^ all the Pencils
hollowa that is> withouc any Rays
in the Middle of them j it is evi-
dent that che Poinc A is feen in
the Place marked z by che extreme
Rays HR, and a few others near
them, and in the Place' marked 3
by the Rays HQ, HN, whereas, o-
merwife ic would have been feen
only confufedly in A by the mid-
dle Rays P, and chofé which íur-
lound them. And becaufe the fame
Thing happens in every other Poinc
of che Arrow, ic íhóws that ic
oughc fo to appea.r double, that
wneii theright Hole DE of the o-
pake Body which covers the Pupil is ,
ftopped, the lefe Image OQ, and
the Arrow on che right Side dif-
appear 5 and if ihe lefe Hole he fíop-
pecl, the right Image and lefe Ar-
row difappear. But if on the other
Uand, we fuppofe the Eye to be oíd
and fíat, ío that the Botcom of ic is
not OQNPRj buc very near GYH,
and that che Rays of every Pencil
arrive ac the Botcom of the Eye be-
fore they are colleiRed inco a Poinr^

the Arrow will be ícen double again,
buc fo that the Images of it upon
fttmping the Holes by Turns, will
difappear in the contrary Manner to
what they did before. Further, by
the fame Argumenc we may colle£c»
that if there be a great many Holes
inftead of Two, there oughc to be
a great many Images of the Obje( 3 :
feen. Eaftiy, Why che Body which
appears double in chis Manner, ap?
pears to be edged wich Colours alio,
may te feen ia che Notes m Chap,
xxvii. u 4 rt, 65'. tovoards the End,

Thlrdly, Why^ if there be tvoo Tab. X'iTU»
Candles A and Ti fo placed, that Fíg. 4.
through the Hole S, only the Candlt
A can be feen voith the right Eye
Fi and only the Candía 3 rorth the
lefe Eye D 5 when both the Eyes are
open together, is there one Candle only
feen, as if it were in Hj btet the
Candks mujl be both of the fame
Heighth, and at the fame Time no
opake Eodies muji be feen with which
the trne Places of the Candles A and
3 niay be compared? The Reaíón
hereof, is, Thac becaufe one Candle
only can be feen by each Eyej and
one Eye only makes a very bad
Judgement of the true Diñance of
Obje»Sls,' each of cheíé Candles are
therefore feen nearer chan it really
is, the one in che Line AF, and ’

the other in the Line BD, and there-
fore they feem both to uní te in the
common Place H as if they were -
buc one.

4

2^4

RO H AU LT' s SvsTEM Part],

it follows, that when the Cataradi is taken away, the
whols Chryftalüne Humour is taken away, or at leañ,
is made flatter or lefs convex than it was befóte:
Now if this Hutnour be leís convex than it was befóte,
the Rays which the Eye receives frotn every Point of
the Objeót will not be fo much refradtedj or will not
incline lo much to each other, as to be able to unite
together when they come at the Retina; and this muft
make the Viíion confufed. But this may be remedied by
the Help of a very convex Glafs, which makes the Rays
that were before diverging, become converging when they
enter into the Eye.
t.mymjie 8. VPhy do Divers, ’whenthey are under Water y fee d
tmfafídiy:, Things cofifufedly, unlefs they make ufe of very convex
Glaffei ? The Reafon is, becaufe the Rays of Light
which come to them from the Objedt, are very little
refraded in paíling out of Water into che Aqueous Hu-
mour of the Eye, fo that tbole Rays which come from
the fame Point, are not united together when they fall
upon the Retina; and this is remedied by very
Glaffes.

9. Laflly, Whence is ít, that if vje fejut one Eye, ani
bai tntmtiy vúth the Other, upo?i a íinall ObieB, inhich ¡s

.upona fmaU Vijta?$ce juppúje; vje cannot at the Jame 7ífm

Qtjed, me fee another fenall ObjeSl-^ •which is at a little moyo than
‘^thlífmaü ^ Difiance from it ; though vie can fee it, if it
oíjea mhkh be a little nearer, or a little further ojfl The Reafon
iywsneat ¡s, becaufc when this other Imall Objed is at the Place
where it cannot be feen, it impreíles the Image exadly
on that Part of the Botcora of the Eye where the Oji-
tick Nerve enters in, and where the Separation of the
Capillaments of this Nerve is made, in order to fpread
themfelves every Way, and cover the Bottom of the
Eye ; fo that this Image has no EfFed, becaufe it does
not fall upon che Extremities of the Capillaments of the
Optick Nerve, which is neceíTary in order to Sight, as
has been before explained.

to-Thatitis 10. There are innumerable other Queftions upon this
fmetimes Subjed that might be asked; but they who rightly
^tlke'°thé‘ underftand the Nature cf Vifon, will find it no great
Tains to find Difficulty to refolve themfelves, and the Pains which
csttheTrath. thgy f^ke in finding out the Sblution of them, will.

make them have a clearer Notion of them, and render
them more familiar : And as to chafe who are unca-

pable

Chap. 35* Natural Philosophy.

pable of underftanding them, or who will not be at any
fainsj it is to no purpofe to attempn to fatisfy them, by
explaining a greac Number of Queftions. Wherefore I
iliall here conclude this firft Part j which is l'ufficienc to
content all reafonable Perfons, and to open the Minds of
fuch, that they tnay for the future proceed in a righc
Method of difcovcring the Truth, and avoiding Error>
which are the Two Things we ought principally to have
in View in all humane Sciences. For the Exaótnefs and
Improvement of Reafon, together with fuch a Freedgm
and Openefs of Mindj as may tender it capable of judging
fincerely and impartially, and of clearing it felf of all
Difficulcies, are incomparably more to be valued than
the Knowledge of all the Sciences in the World.

^he End'vf the Firft Part,

BOOK3

B o o K S prhitei for J. . and J. KN'APtoirJ

BOOKS Wrimn hy ] ohn Clarke, D. B.Dean of S3.mxd.

An Enquiry into the Caufe and Origin of Natural and Moral
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prefent State and Condition óf Mankind is coúíidered and ex-
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founded by the Honourable Roéeft Boyle, Elij; ín i Vols,
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TheTruthof the Chriftian Religión. In SixBooks.by Hupi
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Aninftitution of Fluxions; Containing the firft Principies,
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MathematicalSchool in Chríjl’s Hofpital. ad Edit. revifed, cor-
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A Defenceof Chriftianity from the Propheciesof the Oíd Tef*
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Kind of Proof, advanc'dina late Difcourfeof the GrounJs and
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Refleñions upon Learning, wherein isfhewn thelnfuffici-
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I'a J3 . n .

1

Tab.iu.

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B o o K S J. aiul ]. Kk APTOS?

B O O K S i V S A M I F I r , . „

Z>. A AV.7- -■ J.imcs’i Wcflminftcr

fr-.njfd t, !- I A '.i 1 . S .;</,/ I o II N K N A 1* T ON

ai f'.-í Crown ... .y,-. ICiurs-Church-YarJ. ’

A nilcourfcconfcrningrlv Attr.iuu, cf Gti. iht

Oü.i.tíit.onsol Njtcia:Rd.vK.i.. and ihcTruth andCcr-
ot (he Chr , lii Anfwcr to Mi.Htik
ibc A„j„or el th, Qrtclt, ef Rttfin, and other Dt-

m'/'c P Religión. Being fixtccnSo-

mons Prcached .t the Cathedral-Cliurch of St Paul, intSt

I^c<fli;rc Founded by the Honou*
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cation o ibem to Chnjl. Therc is alfo added, An Anfwer
I?icc h>cttcr, concerning the Argument a priori.

M ^ Whereín for the^r-

cr LndernandingrhcSacrcd Hiílory, the wholeTextandfara-

Fk P^ínted in feparate Columns ovcr-againíl eacho-
cr. ogether with crirical Notes on the more difficult Palía-
les. ^ cry uícfu] íor Families. In two Vols. The FourthEii-
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F.íTax's on BapriTm, Confirmation, andRf*
Ff íullInílruétionsFor a HolyLife, witli

^ ortations, cTpcciaJJy to youDg Períbnsdrawn from

he Conlidcrationof theSeverity oF theDifcipline of thePri-
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A Charitabie, 1 1 6 For y/, bound.

TñiífnUr n ^^'T>odv:eJ^ whcrcin all the Argumenta in his

ef/fV / 1 the Immortality of theSoul, are par-

Fernír^J?/^ aníWcrcd, and the Judgement of F^itherscon^

I truly reprefented . Together with F 0

Author ofRemarks on the LettertoUf-
of a Pn 1 are added, fome Refledions onthtff

wMrf I or the Defenfe of Mi/m’s

Cannnof Writings of thePrimitiveFathers, andtlie

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Witha^ An ^ Natural Philofophy and Religi^ *

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Univerfirv^ NeceHity, From a Gentleman of the

Alfo ’ with the Doaror’s Anfwers to thei^’

ry EntituPd, APhiloíophicalEnq^^

^ ^^«cerning human Liberty

Sfiveoteei»

BOOKS printedfor J. and J. Knapton.

Seventeen Sermons on feveral Occafiona. Particukrly, OÍ
theereat Duty of Univerlal Lovc and Clurity. OF the Go-
mLent of Paffion. Difcourfes upon Occafion oF thcPW.
nf St Peter beins the Rock on which Chrift built bis Church.
Of theFaith oF Abraham. OF being the Bread oF LiFe.
OF the Original oF Sin and Mil'ery. OF Eleaion and Reproba-
don being aParaphrafeon Rom. ix. The Ptt/é»# LiFe a State
a ProbaL in order to a Future LiFe That C¿r(/2 s Admom-
tionstohis Apoftles, belong univerfally, to all Chnftians. The

^^actbiRohaukiPhyíica. Latine vertir, recenfuir, &ub«i°-
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fophia maximam partem hauftis, amphficavit & ^

Chút, S.T.P. Accedunt etiam in hac^aftM

aliquot Tabute teri incife; & Annotationes multum íuut

Optice. Latine reddidit, S. Ckrke, S.T. P^

C. JuliiCscFarisqusextant, accuratiffime cum Lbrsedi
& MSS. optimis collata, &c. Accefferunt AnnoUtiones 5.

^%tslÍ¡rlBoar¡r.e oFthe Trinity. In ‘hreeParta Where-

in all thcTexts in theNewTeftamentrelating to

and the Principal PaíTages iri the Liturgy ^ second

Lnp^nd} are colle6tcd> coinpared, and exp ai

Won.Vriccós. Reñor oí Coíes¿’Mh ia.

A Letter tothe ReverendDr.PTf///, p .

Ldceflerfhire, In Anlwer to his Remar of aft

A sltmonptcichriitthePan^'^liuf^^^^^^^ of

aSvIlLi',';.

'ants, 8'z;o.í)r/Ve4</. , ^ rhe Prophecícs ín

A Difcourfe concerning the Conripi Chrift. Be-

ji^cOld Teftament, ^nd the Applica n) ^ j^emonjlration of
‘i^gan Extraíf from the Sixth Ed jsadded, A Let-

fheBeing and Attriíutesof God. Secand Edition

íer concerning the Argument, ^ the annexed Letter^

— .V,.. This Difcourfe

eompleats the former Editions
Priceu. , ,

Remarks upon a Book, entitu e >
concerning Hnmfin'Lii’értyj Price

b ’

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